I bought my RC this month last year and have put over 11k miles on it all riding within the New England area. To say the least, I love riding it. But somethings I get dreaming on what my next bike will be. I know it will be something in the middleweight class(636, 675, 750 etc) because I simply don't use the power and would like something that I can wring out on the street.
Over the winter I invested in getting my forks reworked by DK, and getting a Penske double clicker shock. It absoultely transformed the bike and it has become planted and confidence inspiring in the corners. Since I just put a decent amount of money on suspension, I wouldn't want to lose out on a nice setup on my next bike. Many of the newer bikes come with Big Piston forks.
So how do these forks compare to the RC's forks that have been revalved? Have 13 years of advancement with OEM equipment exceeded forks that are 13 years old but with updated internals? Anyone have experience with both?
Your first paragraph is a bit confusing. Are you saying that you aren't able to wring out the RC51 on the street, but you think you'll be able to do that with a middleweight 600cc-ish bike? You won't be able to do any more wringing out of those bikes listed than you can with an RC, more likely even less so. Most of what you have listed will have nearly the same power as the RC51. Maybe a 10hp difference.
If you want something you can really wring the neck of, and if you'll let your ego out of the way, a 250 is the perfect bike for that. Most people discount little bikes before they ever have the chance to ride one in anger.
On to the forks: A well setup (weight, valving, etc) set of forks will almost always outperform a set of bone stock forks.
When you have DK rework your forks, you aren't "updating" the internals, you are replacing them with better quality components. Even if you step up to big piston forks, you'll still have relatively the same quality level of components as the stock RC51, but with a different type
of component. Kind of like comparing a well setup damper rod fork with some emulators to a bone stock set of cartridge forks.
If you are asking wether or not you'll have to spend a couple grand to get the same performance out of BPF's vs what you have now? Well that is up to you. I'd go ride a bike with them and see what you
think. Some people will swear that every bike needs this or that to be remotely ridable. I was just fine doing mid pack, level 2 trackdays on completely stock RC51 forks, yet some think they need 20 or 30mm cartridge kits to ride on the street.