Depends on your skill and the bike you've got. For example, why dump a lot into performance mods on a 600 instead of buying a 1000 instead?

So for a newb like me I'm going to keep my 600 for a while and do mostly aesthetic mods like the ones I've done so far. I've added a new pipe which could be considered as a performance mod, but I really did it for sound quality, which is aesthetic. I have no intention of adding a PC on this bike. Doesn't matter what you do, a stock 1000 will beat the hell out of a fully modded 600 any day, so what's the point?

If I had a 1000, I'd still start with aesthetic first, because you don't really need that much in the way of performance adds to a 1000.

 

i take it you're referring to just dumbo straight ahead speed...then liter yes...
but as far as cornering, track, etc...a liter will not beat the snot out of a modded 600 anyway, it's the better rider and the person who knows the bike better.
plenty of people on 600s eat up liters at the track...and the fastest person at the track is not always the one with the liter bike.

suspension is considered a performance mod...and liters don't necessarily have better suspension than a 600.

 

Here's what I did to my race bike and any other future race bike. Not much mods on the street bike other than a slip-on exhaust, power commander and steering damper.

1. Respring and revalve the forks and shock
Replace brake lines with stainless steel and change stock pads to Vesrah
Steering damper
520 conversion and change the sprocket sizes(-2 front, +2 rear)
Replace the exhaust and replace stock air filter with a K&N
Power Commander with custom mapping(s)
Race tires
Replace stock body work
2. Rearsets
3. Replace the stock front brake master cylinder with a Brembo radial master cylinder

 

order doesnt matter, just get it all doen in the end

 

The difference in speed, top end litre bike all day. With the right gearing a 600 keeps up. The 600 is lighter and easily out handles the 1000.