I'm not too sure.

I know that the coolant is restricted significantly at the head gaskets. If you compare the coolant ports to the head gasket openings you'll see what I mean.
But! That is done on purpose to raise pressure at that specific point. Higher pressure means a higher boiling point. That keeps air pockets from developing in the heads.

If your trying to get flow better than stock, you may run into pressure issues.

I was looking to get the system to flow closer to stock since I'm not sure how other or more drastic changes will affect the system.

Edit:

I also forgot to mention that the bypass plate is pretty bluky. A dremel could be used to modify it enough to improve coolant flow, possibly back to stock.
This is another option that dosen't involve a top end tear down. You may just need to take the cross-over off and drain the coolant to do this.

My coolant temperature acted the same way before I blocked the TB passages, even with the 160 stat. :?
The benefit is a cooler intake charge that doesn't have 160F coolant or 500F exhaust gases going into it.
I still think the limitation is the radiator.

I'd bet some of the radiator places online could tell us what the flow and total temp drop across the rad you have would be. Then you'd know w/o a doubt if it is the limitation. IIRC your rad isn't thick..but it'* tall.

They could in theory if the flow was constant but there are too many variables.....engine speed which relates to water pump speed, air temp, air flow related to vehicle speed, engine load, etc....

The core only has one row. On my Firebirds, I changed out the stock 2 row cores with 4 row cores and have no problem maintaining 160 F under any condition whereas before there were similar problems.

There is plenty of room to double the size.

Blocked coolant ports in about 20 S2 motors, the ONLY ones I've ever seen with any issues aren't "burped" correctly. 3hrs of idling mine sits at 174* with NO tstat on 90*+ days. Electric water pump, ect. The only time it get warmer is after a hard run (100mph+ for a mile or two).

If your seeing cooling problems-try burping the system correctly. My .02

Idling is not the issue.
Running under load at 90 mph for 30 minutes in 90F weather or idling after that IS

.....and just what do you call proper burping technique :?: