Spareparts you obviously don't have a clue and not ridden a 1290 SDR...Racing on the street is for idiots as fast on the street with one leg in the cemetary and the other one in jail is maybe 7/10th of fast track pace.Yes at CSS you can race they have a class for that but even in level 4 you don't ride with or behind a instructor but do your own thing and have a transponder giving you lap-times.So even if you are not racing you can still go as fast as you can and have the times to prove it...
As for the GS being perfect with 170-ish hp and 17" wheels let me explain to you...Of course a S1000RR will still be 40 kilos lighter and 40 hp more powerful on top and thus faster on the track.But this is not the point as we talk fast road-riding where upright seating position,high TORQUE and Telelever are bonus points for obvious reasons and 170 hp is enough to go fast and have an exiting top end unlike the KTM being twitchy and slow steering and wasting all that straight line power in wheelies...
Let me explain the benefits of 17" wheels for the GS...They obviously build up less gyro at higher speeds than 19" wheels as also you can get real good rubber like Conti Sport Attack 4 or Metzeler M9 Sportec not available in GS sizes if you want to go on a track day or ride really hard in summer where the sportiest tires in GS sizes could get to their limits.
Also the geometry of the bike could be set a little more on the stable side which would be beneficial in high speed stability and fast corners and due to the 17" wheel building up gyro the bike could still handle as quick as the current GS does.At least in theory...

Because if you slap 17" wheels on a current GS you gain looks but loose ground clearance which is never good on a bike where you can already drag the pegs easily (GS without sports suspension) and of course you loose trail which decreases high speed stability (especially if you run with bags).
Anyways that is how I see it and from my experience after almost 40 seasons in the saddle !