I've had an ally flywheel on my 748 for about a year now.
no downsides, still ticks over well, doesn't stall, engine braking is stronger and the thing revs up quicker...........
What could happen is that the reduced inertia could exagerate a poor fuelling setup.
Have to agree with Shazaam on all his points ;)
Cost about a £100, fitted in a couple of hours comfortably. I think it's one of the best mods you can do :D

Does a SPS have a lightened flywheel as standard?
Tim

Just a couple of observations while totally agreeing with Shazaam!
A single cylinder engine on compression stroke with a shut throttle is basically trying to stall, this effect is reduced by either adding extra cylinders with different firing orders, hence multi-cylinder bikes having less engine braking for a equal compression ratio, or as already mentioned increasing the inertia, ie a heavier bottom end, hence race bikes using lightened and ballanced crankshafts for better engine response.
On a race bike more engine braking is often a good thing, especially on a twin when combined with a slipper clutch, as it is easier to change the bikes direction when the engine revs are lower due to the gyroscopic effect of all that mass/inertia trying to travel in one direction.
There was something else I was going to add but its slipped my mind at present, poor old brains on over drive preparing two bikes for the TT, 48hrs from now and I will be on the Ferry.

No. The SPS or any other street bike to my knowledge did not come with alloy flywheels. Surely, someone will correct my error, though.

How much engine braking do you get with a slipper clutch or can it be set up to only work on or close to wheel lock-up ?

Would a slipper clutch be able to cope with 6000miles a year or would I be spending a fortune on clutch plates ?

Had a 900ss which made strange noises when setting off (sounded as if chain was jumping the cogs.) and this proved to be problems with the clutch (standard clutch)
and was repaired under warranty because it was only 2 weeks old.

Slipper Clutch...
 

"How much engine braking" is a difficult one to answer - lots!!;)

The slipper clutch is designed not to lock up the back wheel when you've changed down through the gears.

So in theory you'll get maximum engine braking without locking up the wheels. As I've been having false neutral issues anyway, I've found myself going down a lot more than normal without meaning to (in my anxiety to get any gear!) and found that the clutch just releases (slips) as much as it can without locking the wheel up. (am I repeating myself);)?

As for mileage, no reason why it shouldn't last a long time, unless your racing every weekend! - So depends upon use really

Tim