is 11.2:1 compression too much for 91 Octane

IMO it's boarderline runnable that way. If it's a std 505" kit it's running flat tops to get that high. 11.2 isn't that high. My 505 packages are usually 10.8:1 - but the cam is another 10° bigger at .50, and the LSA are a few degrees tighter.. If it uses dished pistons they will be close to the head, and quench will take care of it. If it's flat tops and they are .040 or more away you're boarderline with that cam. You might be able to tune it out like AutoX said. Sometimes ignorance works in your favor... But you might need to put in a larger cam or thicker head gaskets, so factor that into the choices. I stay away from additives. It seems too risky that you'll end up needing it somewhere and not have it...lol.