Is there any benefit to milling heads down say .030 on a boosted car? Is this possible to do and still have the LIM line up properly?

Don't really know so I went on a quest. Can't find anybody that has done it, or do I see it mentioned as I perused alot of sites that offer various Staged 3800 Heads.

I see it changing the geometry of the head LIM connection point. Also, I really see little point in doing it. You can drop a pulley size to do about the same thing. Or better yet, build up a L36 or L26 block with a top swap. The L67 has 8.5:1 compression, and the L36/L26 has 9.5:1 compression.

I think on boosted applications you want to keep as much material on the deck of the head as possible for strength. I know it'* been done but the boost levels weren't very high.
I know of at least one person who milled 0.030, used stock thickness head gaskets and didn't modify the LIM. They didn't report any issues. I'm doing 0.040 and thinner head gaskets so I had my LIM milled on all 3 sides accordingly. But the engine isn't going to see any boost.

I know the boost wouldn't increase. I guess I was asking if a boosted car would benefit from the possibly slightly increased comp ratio

IMO, What your going to get compression wise milling heads is for serious things like when a tenth of a second counts for money. And you routinely blow powerplants. To do it to a DD is a ridiculous trade off.

I have some milled heads that are about to go on! They're 0.010" shorter, and I have the thinnest Cometic MLS gaskets, which are about 0.010" thinner. Increasing compression actually is said to be a good idea for stock Gen III M90 setups. The reason is that when you drop a pulley size, you increase heat. Increasing compression effectively lets you have similar power with a larger pulley size, less heat, and therefore more oomph. It puts your M90 into a more efficient RPM range while giving you more power. I actually prefer the idea of increasing compression over dropping pulley size, but it'* a whole lot more expensive and is relatively a more permanent change.

You would have to have the LIM machined as well, yes. If you don't have it machined, it will still fit but the ports won't line up. You'll have some "overbite" and "underbite" on the LIM-to-head ports. Since L36 guys with an extra atmospheric pressure increase top-swap and get about 10 extra horsepower, an L67 engine having its compression increased isn't actually so bad or so risky as long as your tune is good, you have good fueling, and all that other good stuff you have to go with when dropping in pulley size or going to a larger/more efficient supercharger or turbocharger.

Also, you should get shorter pushrods to accommodate the new valvetrain geometry. But as Venom said, no need. And as I might have said, you could just drop in pulley size and tune accordingly.