Few things from my experience - pretty much the hardest I could get a standard jaguar to sound in terms of distortion and balls is the track 'Livewire Pariah' at - http://www.myspace.com/moodmarket
As you can kinda hear, its great for clean sharp tones similar to bands like Interpol, and with enough gain it can give you a bit of trebly bite, but it dies off pretty quickly.
You can get hotter singcoil pickups for regular jags, buy a hh jag (which only really come in three colours, two of which I havent seen made in a while, but pop up on ebay (black, blue and candyapple red)). They have the same scale length, but no Jaguar tremolo which is my favourite part about a jaguar.
The other option is as Nirvana freak recommends, look at humbucker replacements. Problem is all the humbuckers with singlecoil sizes sound very hot: 80s and screamy like Poison, and not retro grungey like Nirvana. For an authentic sound you have to route (as in chip the wood of the body away) so that you could fit standard quality humbuckers in the guitar - a nightmare if you ever want to resell.
Replacing the pots to 500K would also take away some of that treblly bite.
Standard Jags come in olympic white, candy apple red, sunburst, lake placid blue, black, and by far the best colour Surf green - occasional runs of other colours like antigua, sonic blue, sherwood green and sea foam green and even purple pop up on ebay from time to time.
If your gonna mod, buy a japanese jag as its not worth as much...
