Ol'89r,
There's absolutely no point in adding a switch to toggle back and forth. Once you remove the grey wire, you won't want to put it back in. It makes the throttle more responsive, but much more controllable. It is especially important in that it allows you to run a gear higher in low traction situations. If it starts to bog, you can pin the throttle and not have it puke on you. It will just hunker down and keep pulling. After having removed the wire on my WR250, I'd never, EVER consider hooking it back up. If you do a search over at Thumpertalk.com, you find tons of info on the grey wire, and also another poster who had the same idea as you.
Yes, the grey-wire fix only applies to WR's (all sizes), and then only on US bikes. It has the biggest effect on the littlest WR, and is the simplest and cheapest (as in FREE!!) way to unlock more power in your bike and get rid of the annoying flat-spot that appears after you remove/trim the throttle stop. It's there because of the emissions/sound requirements the EPA has imposed on off-road (not closed course) bikes. If you pull the grey wire, you MUST remove the baffle from the stock muffler, otherwise it won't run worth a darn. I even tried modifying the stock baffle (swiss-cheesing it with a bunch of holes in the end plates) but it was still too restrictive. Go with a Vortip, or an aftermarket exhaust if you need it quieter.