Assuming the pistol is reliable, modifications in order of usefulness:
1. Trigger. A 1911 with a good trigger job will have the best trigger you can get in a combat handgun, period. Better trigger = easier to shoot well. This is probalby the most useful upgrade to a 1911 and relatively cheap (say ~$100).
2. Sights. Depends on what comes with your gun, but mil-spec type sights suck. Really, really badly. If your stock sights stink, then better sights like Novaks or Heinies will make it easier to shoot well. Adjustable sights are generally a waste of time for most people IMHO. Generally not too expensive.
3. Barrel. This again depends on how your gun is stock. Kimbers tend to be very accurate and probably wouldn't need more than a $50 muzzle crown to ensure Ludicrous Accuracy. If your pistol's (as distinguished from YOUR) accuracy sucks a fitted Bar-Sto barrel can easily bring your pistol's accuracy into the 1" @ 25 yard range or better, but between the barrel and fitting could set you back as much as $300. Almost all accuracy comes from barrel quality and barrel-to-slide fit, so while $300 sounds steep bear in mind that this alone will make all but the crappiest 1911 exactly as accurate as a $2,000+ custom piece.
4. Everything else. Aside from the above, everything else done to a 1911 is pretty much gravy. Extended thumb safties are more comfortable to maniuplate than stock pieces, and beavertails do improve the ergonomics of the grip for most people. But functionally none of the other stuff done to 1911 pistols makes a bit of a difference from the point of view of how well it pokes holes in pieces of paper.
I did pretty much all of the above to my Colt Delta Elite and am very happy with the results. The key is to have a good pistolsmith do the work. You can read my write-up on my pistol here:
http://www.geocities.com/mr_motorhead/deltabcp.html
, Art