I have worked on a bunch of these engines and never seen a seat drop unless it overheated first.

The later heads bolt on with pushrods being the minimum extra parts needed. The intake and ex manifolds are good to do at the same time.

I came up with 11.5 when I actually CCd everything I had sitting here. That is only a little more than an apache 392 so I think it could live on premium. You can try thicker head gaskets but in my experiance and many others the higher quench distance is more likely to cause detonation than the higher static compression. The late hemi chamber is very detonation resistant and the plastic intakes keep a lot of heat out of the air charge. All this adds up to a very compression tolerant design. The extra compression will improve MPG some and help offset the cost of premium though probably not the entire cost difference. You could upgrade to a 6.1 cam and also bleed off a little compression for cheap as well as help hp.

The short block is diffent enough to make swapping the short block not worth it, the VVT uses a different timing cover and will make some headaches in the swap.