There's actually a bit of...
There's actually a bit of storage space beneath the RC51's tailsection cowl cover. Twin mufflers on the SP2 version were made from thinner-walled steel, dropping almost three pounds from the previous exhaust.
As for the exhaust system, Kyle says their extensive in-house testing has shown that very few if any of the full exhaust systems make any more power than the dual slip-ons offered by Sato and Moriwaki and each saves a total of 10-11 pounds. Other popular choices among riders include Erion, Jardine, and Akrapovic. While most of the 2-into-1 exhaust systems do offer slightly higher top end power, it comes at the expense of the midrange that v-twin riders enjoy. The use of high mount exhausts means the removal of the passenger pegs, so choose the correct setup based on your intended use of the bike.
Most tuners opted to leave the stock air filter but disconnected the internal "flapper valve" in the airbox, which was there for EPA noise emission reasons. By simply disconnecting the vacuum line feeding the flapper, it is left in the open position which helps remove a dip in the power curve at the 4500-5500 rpm range. Another popular trick is to defeat the factory "soft rev-limiter". In stock form, the soft limiter kicks in at 9000 rpm and then the hard limiter waits until 10,200 rpm. By allowing the bike to rev freely all the way to the hard limiter, the combined result of the exhaust and EFI tuning nets a nice 8-10 horsepower gain on top.
The superbike racing intentions...
The superbike racing intentions of the RC51 can be seen in the brake caliper hangers, which can be swapped out to allow for larger discs or different calipers, and the external compression valving cylinders that permit easy valving changes instead of disassembling the fork leg.
Perhaps the biggest improvements on the bike come in the suspension department. Although the bike came with a nice setup, according to Kyle it is a big compromise. Due to the fact that street bikes are set up for solo and dual riders, the suspension is quite progressive. If you are willing to set up your bike for one weight range, it allows you to optimize a suspension system that is much more linear. The most popular change is to add an Öhlins fork kit. Since the RC shares the same basic fork as the CBRs internally, there is a lot of data and parts available to set it up for most any rider. The Öhlins kit includes springs and internal re-valving and goes for about $650 from Kyle Racing.
On the rear, one of the least expensive fixes ($349) is to simply change the rear suspension link with one that is less progressive. Kyle Racing makes their own link and not only does it address the progressive nature of the suspension but it also raises the rear slightly which helps the RC turn a little better. Out of the box, the RC51 is very stable, but it also steers a little slow. An advantage of this design is that a steering damper is not an absolute requirement. Some riders have it but they do so mostly as an insurance policy, not a necessity.
Unfortunately the rear shock itself isn't the greatest (even the upgraded SP2 unit), with overly stiff high-speed compression damping rates that can't be adjusted out externally, and delving into the internals is a major hassle. Kyle says the only real solution is to replace the stocker completely with a full Öhlins rear shock or similar quality aftermarket unit. It is a big expense but one that riders never regret after doing it.
For rider comfort and increased ground clearance, rearsets are also common for the RC. Popular brands include Sato, Vortex, Gilles and Arata. The installation of new clip-ons can slightly raise the bar height and take some weight off the arms and wrists, which often helps a lot over the course of a day. For those riders trying to shave off as much weight as possible, aftermarket wheels are a great place to shed some unwanted pounds. BST and Dymag have carbon fiber wheels available if you have the bucks. For slightly less weight loss and a lower cost alternative, others opted for forged magnesium. Common brands we found include Marchesini, Marvic and Dymag.
Another upgrade is to replace the sprockets due to the tall gearing. As supplied from Honda, top speed can almost be reached in fifth gear because the bike will not pull redline in sixth. Changing the sprockets from the factory 16-tooth front and 40-tooth rear to a lower 15/41 setup will usually maintain the same top speed and increase drive out of corners too.