is 11.2:1 compression too much for 91 Octane

Wow. My 273 Commando, nominally 10.5:1 compression (probably less at .030 over), at factory timing is right at the edge of pinging on 91. It's iron, of course. I'd say it's a maybe -- you'll just have to try it.

Over bore adds compression. It does not take it away.