You're going to be hard pushed to keep up with the latest Jap stuff on power Mozza, you've probably got 106bhp and that's 600cc outputs these days.
Having said that you shouldn't have any trouble keeping up on a trackday if you're in the right group and can ride well. You'll notice the difference on the straights but you'll have them going in and out of the corners. If you deecide that's not enough then there are plenty of good engineers out there who'll tune the 916 for you but it'll cost a fair bit. Swapping the engine will require injectors/cpu/set-up etc.
Probably the best course if you prefer the look of the 916 is part exchange for a 998S - 136 genuine back wheel bhp should keep you in touch with most Jap IL4s
Check out
www.sigmaperformance.com for lots of tuning info.