I've seen where both the block and head can be machined quite a bit. On my /6 rebuild, planning to equal remove amounts from head and block. I'm thinking that .060 total will produce desired results without having to use higher octane fuel.
Upsides: More compression, better fuel mileage, more power (Mo par', to the cornbread and grits crowd. Self included.)
Downsides: May require higher octane. This point is open to debate, but many say that taking .090 or more out of the head/block combo and you'll need new (shorter) pushrods to have room to adjust valves.
Recommend you check out Slantsix.org . Lots of good knowledge there about warming up the leaning tower of power.