What are the most common mods. for the RC51 from mild -wild?
In order of most obvious aka most common aka least likely to have negative side effects:
Change OEM sprockets from 16-tooth front / 40-tooth rear to 15-tooth front / 41- or 42- or 43-tooth rear = mild, but makes the bike run better, i.e. greatly reduces the "I have to slip the clutch too much from a stop and once I'm rolling I can't find the right gear" factor.
Add a Power Commander = mild, but makes the bike run smoother / better.
Add a Speedo Healer = mild, but corrects the speedo error inherent from the factory and exacerbated by the gearing change under 1a.
Change the OEM rear 190/50-17 tire size to 190/55-17 = mild-ish. Changes the steering geometry to make the bike turn faster. Might need a steering damper after this.
Add a steering damper(?)
Some guys might recommend a new rear shock and / or fork upgrades prior to this point (i.e. "suspension first", but the cost for those is generally much higher than 1 - 4 above. And probably not really needed for street riding. Me, I prefer the bike to ridable first, hence items 1a and 1b at least.
There are more. I got tired.