I know theres little problem from 4" piston as long as the CR is not to high for a bike that will see some long miles but when is big too big of a piston dia. on a say 9.5:1 CR motor that will see some highway miles?
Kags, I've been running a 113 ci s&S in a 94 RK for a few years , it just recently went past the 100k mark for the motor, the bike has a little over 200k on it. The s&s motor has never had less than 10.5:1 comp. The one major problem I had was around 60k mi when I went with a .653 lift redshift cam and started to ride 2 up [ the older I get the longer it is between girlfriends ]. I run a baker 6 speed also and the motor had so much torque I was lazy about dropping into 5th to get around traffic and lost a piston to dettonation. Live and learn. If you are considering running a 4" bore motor in a long haul bike, go for it, we are only here once, , you will never look back. If you go this route let me know and I will pass on the mistakes that I made.