once you turn off all the extras for sp3, it runs pretty quick as well. you can get it down to only a handful of services at start-up. should not see much of a difference over sp1 if you tweak it.
but as bubblehead said, to get sp3 functions, you'll need sp3. it may be possible to track down stand alone installers if you know exactly what you want to add. they did offer a lot of stuff separately for the business side of it. but you'll need to know exactly what you want to add and search for it one by one on the MS site. it may not be available anyway but it's your only shot really that i can think of