Well, I've got the mod bug on the “daily driver” ('98 LeSabre, L36, in sig). Thanks in large part to what I've learned from this forum, I've got something of an upgrade plan coming together. Please comment, critique, and make suggestions on my proposed performance modifications:

Performance Objectives:
-Nothing big; just a little more “pep” in the daily driver; particularly at mid to high rpm’* (e.g., passing on the freeway).
-Little to no loss at low end rpm’*.

Other Considerations:
-Maintain at least 20 mpg in mixed city/highway, and 30 mpg highway.
-Maintain good reliability.
-Be able to pass California emissions and smog visual inspections (among other things, necessitates keeping as “stock-looking” as possible).
-Mods must be “reversible”, and “portable” to the 2004 LeSabre (for if and when I get rid of the ’98 and “inherit” the ‘04 from the wife 5 or so years down the road).
-Must be within reach of a "shadetree mechanic" with hand tools.

What I DON’T Want:
-A race car / drag racer.
-Lopey idle, “bang/screech” shifting, a “tire-burner,” or any of that nonsense.
-Something that needs frequent retuning.

Cost Constraint:
-Counting only car parts (i.e., NOT counting tools, diagnostic devices, programmers, etc.), not more than $20 per hp gained.

Modification Plan:

1. 180° drilled thermostat / Prestone (already done). Expectations: improved UIM life (though minimal performance gains expected; more a "support mod"). Cost: $35
2. 4T65E shift kit (already done). Expectations: crisper, more precise shifting, though no measureable hp gains expected (again, more a "support mod"). Cost: $90
3. Ported LIM (in process). Expectations: higher VE% (from ~70% to ~73-75%, increased torque peak (from ~4000rpm to ~4200 rpm), maybe 5 peak hp. Cost: $40 (already have junkyard part)
4. Modified UIM runners (in planning stages; a “poor-man’*” HV insert). Expectations: higher VE% (from ~73-75% to ~75-77%), increased torque peak (from ~4200 rpm to ~4300 rpm), maybe 5 peak hp. Cost: $40 (already have junkyard part)
5. PCM tune (planning to obtain DHP PowrTuner). Expectations: raise shift points (to ~5800 rpm) and optimize fueling to further increase VE% (to ~80%), etc.; maybe unlock another 10 hp
6. 1.8 Rockers (still determining options and comparing vs. cam options) Expectations: increased VE% (perhaps beyond 80%), increased torque peak (from ~4300 rpm to 4500+ rpm) more high-end torque, maybe another 10-15 hp. Approximate cost: $350 to $600 (depending on need for springs, rods, retainers, valve seals, etc.)
7. Gutted airbox /smooth bellows / K&N Only if needed as support mod. Cost: $55 (already have this, but not needed at current mod level)
8. Throttle Body port or swap Only if needed as support mod (very doubtful, IMO). Approx cost: $150 to $200
9. Power log or port front exhaust Only if needed as support mod (also very doubtful, IMO). Approx cost: $100 to $150
10. PCM re-tune Expectations: raise shift points (to ~5900-6000 rpm) and further optimize fueling, etc.; maybe unlock another 5-10 hp

Summary:
In the end, I would expect to raise torque peak from about 4000 rpm (stock) to about 4500 rpm, increase total hp (over stock) by about 35-40, and spend about $700 to $800 on parts.

So, what do you think?
Are my expectations reasonable?
Anything else I should consider?
Are any of these mods going to put reliability at great risk?

I have all of these (minus the pems as of now), and have no problems with reliablity and still get good mileage. of course, ymmv

it is my daily driver, see sig on current mileage

I think your expectations are reasonable.
I would keep the intake runners in the UIM at least 1.5" since your not looking to move your power band up a whole lot. And try to figure out a way to bell mouth the entrance to the runners without blocking the TB inlet.
For the rockers, you may want to get the ZZP split ratio ER rockers.
Easy Performance did the testing on them complete with VE charts and such then sold the design to ZZP. The tech info article on their site has all the VE and HP results.
And, you would benefit from PEMs but I'm not sure if you have smog specific manifolds or not?

I agree that any of the HV insert versions' runners are too short for my purposes. I'm looking at experimenting with lopping off maybe 3-4" of runner, probably cut on a diagonal as suggested by the red lines I added to this pic (modified from a pic swiped from easyperformance website):



ER rockers are at the top of my list. I wonder how safe / reliable these would be with stock rods, springs, retainers, seals, etc.?

I do have an RPO code indicating "CA Emissions"...I don't know if that means there'* a difference between my exhaust manifolds and any others. Anyone know more about that?

I wouldn't be surprised if the level of mod I'm looking for doesn't require exhaust work...especially since my chances of opting for a cam swap are nearly 0%.

That length seems pretty good. It'* just the abrupt transition from open plenum into the runner that I'm worried about. It may hurt flow..
Stock pushrods will definitely be up to the task and the rockers were designed for stock springs and retainers. But, increasing the rocker ratio will always put more stress on the valve seals and guides. (Unless done with a roller tip) You shouldn't have any trouble with this added stress though. Many have added these and modded 1.9 rockers without problems.
I know I've seen the "I want pems but what to do with smog manifolds" issue over on cgp but I'm unsure of the details.
Pems are great for even a stock car. The front cast manifold has horrible casting errors that slow flow and the rear tubular manifold can also be improved upon.