If you are at 150-160hp on an RC51, you'll have bigger problems than a factory pro shift kit can handle. That, and hopefully by the time you dump that kind of money into the RC, you'll know what it is for.
You can read all about the kits here: http://www.factorypro.com/products/s...ctory_EVO.html
They basically make the shifting feel better, and get a more positive engagement to reduce missed shifts, false neutrals and the like.
I don't know what bolt on you can get for the price of a quickshifter that will give you 5hp, depending on what you have done already. A well done dyno tune will be the only thing to net you 5hp for close to the same price of a quick shifter. If you are planning on racing, you should have a quickshifter anyways.
If you are already in the 135 range, you probably have exhaust, PC or the like, and an airbox. Much more than that and you are looking at headwork, cams, valvetrain etc. The work is easy enough to get done, but won't do as much as the other stuff. The other stuff is very hard to find still, and will cost you a small fortune when you do. More than 140 and you are chasing unicorns down a road paved with your hard earned money.
Just my opinion though. I haven't built anything crazy, but I've seen a couple well documented build threads that demonstrate what I've said.