When I do incline and I don't do flat I can end up putting up as much in the incline, and when I return I can't do the same on flat. It varies a lot from one to another, some can incline more. Your case is probably due to doing flat for a long while and just starting up on the inlcine. You don't have to push so much with your legs to the extent that your ass is flying off the bench, remember that you're stimulating a muscle group for a response, not preparing for a meet.
Some that are "powerbuilding" will say otherwise, I've known a couple of guys who do powerlifting style on some movements for physique development, they have PL background though. Heck, I knew a guy who is 19 and can bench 5 plates, obviously this is powerlifting technique, but his chest and delts were pretty big and solid. Feet have to help your body be stable, but no other than that.