the r6 throttle has the same outer diameter as the stock 250r. You can see the difference where the cables attach to the tube.
They are the same length!
Your only clearance issue is getting the black cable guide to fit underneath the lip on the throttle tube.
If you can get it like this, the stock housing of the 250r will go back together and sit completely flush. My throttle cables are adjusted by the mechanic at the local dealership so if yours are adjusted correctly it should take no cable adjustments!
There's a plastic washer on the r6 throttle tube, leave it outside the housing or cut it off as it is not necessary on the 250r.
Now time for some new grips.
Also, when you order the the throttle tube try to order it without the stock r6 grip as it should be cheaper.