I have a '93 KLX650C and have used both the OEM longer steel swingarm and the shorter R model Aluminum swingarm. I know you have an R model, but for anyone else that is interested you can't just put an R model swingarm on a C model. If you do that your rear tire will contact the C airbox at full compression. I verified this in offroad testing, ha. So I also installed an R airbox, shock and linkage - subframe modifications including battery relocation were required - and laced an 18" rim to the OEM C model cush hub. Long story short, none of that affected highway stability as far as I could tell. I've found that highway stability is just tied to what type of front tire I am using. If it is a 90/10 ( for example a D606) I get headshake on the highway around 70 mph with either swingarm. If my front tire is a 50/50 my bike feels completely stable on the highway with either swingarm. BTW, if I am running with a loaded GiantLoop Mojavi the headshake problem with a 90/10 front tire goes away - I tested that up to 95 mph on the highway as well.