So I've been to a couple of track days this year on my 919, its pretty much stock except for my SS brake lines and some cafe racer style handle bars. I'm at a point now where I'm thinking of having a dedicated track bike and another bike to commute/go around town on.
As with anyone money comes into play here. I can't go out and a new track bike and finding a reasonably priced racer for the track is challenging. So I'm toying with the idea of upgrading the 919 to use it solely on the track. My basic logic is that best case scenario I can sell my 919 for about 4,500, more if I'm lucky (07 w/ 11,000km). Decent used RR's are selling for around $7,000 (for a good deal), then I'll probably be spending money to upgrade it...
I can upgrade the 919 in stages, I think that for the track the first things to improve upon would be the suspension. New springs, new rear shock and some sticky tires.
After that performance exhaust with PCIII and new air filter with a trip to the dyno.
Somewhere along the line add in a bigger sprocket for quicker acceleration maybe some pazzo levers if I feel fancy.
I'm handy enough with a wrench that I can probably do most of this work myself, and it would be a fun.
WT is this worth it? Can the 919 be turned into a decent track bike, or would it be better to cut bait on the 919 and go for a racer?
It really depends on how you qualify "worth".
If you want to improve it to a reasonable upper limit, and still have it as a legit naked upright street bike good still OK for two up riding, then I'd say yes, a case can be made.
I did over 1200 kms of track day mileage last season on my 919 last year, and will end with about 600 more this season. Mine has had the front totally reworked with springs and valving, and I have a Penkse 3 way rear. Running Michelin 2CT for street and track tires. The bike is nicely sorted out, and can be hustled around at Race City here in Calgary. I get blasted every lap on the long main straight by all the 600s out. Intermediate Group rider. I reel in lesser riders in the turns. Equal or better riders get away in the turns on bone stock 600s. If you want to be in the mix with the 600s, you need to be on one. They have more power, weigh far less, have superior geometry and chassis characteristics, and are way more nimble, especially in quick transitions from side to side. 919 front geometry is not bad overall, very stable, stiffness is OK for street tires. The overall chassis does have a very significant limitation though, something that little can be done about. The swing arm angle is only about 8 degrees with rider on. The anti-squat is poor as a result, and the only possible compensation is to dial in the low speed compression damping. The ideal swing arm angle is over 12 degrees and less than 13 according to Traxxion. True, you can lengthen the shock to increase the swing arm angle, but you'd have to go too far to get the swing arm angle into the target zone. On top of this is the simple unlinked swingarm setup.
I'm really happy with what I have got my 919 developed too. But as happy as I was with it as a 919, I decided to get a dedicated track day bike. A rebuilt written off 07 GSX-R750 now sits in the garage. I still take out the 919 because I love riding it on the track and it reminds me of my ancient CB750 - aside from having way more go and handling way better.