hi dude,I have a set of these heads and i asked about the same question ,what I learned is as follows.
These heads will produce in the range of about 10.5 to 1 compression ,you must be carefull of detonation. adding about 10% race fuel,you can keep this from happening. you can fatten up or thin out the head gasket enough to get you back in the 10.5 to one range.
there was no response as to horsepower, but i'm only guessing that low end torque would be incredible , but would pi$$ the bed at maybe 4,500 rpm as per air suffocation.it would starve for flow.
from reading quite a bit about G.M. heads , you can't port them out big enough to be effective . using vortec or even common 882 castings (2.02/1.65)if done,maybe more,will flow much better than any 305 head.
maybe this combo would be ideal for say a 4 wheel drive truck, lots of ball$ to pull loads, maybe a 4 speed.
hey you real experts out there , let me know if any of my information is flawed, as i have only been learning this for a while myself. mike