The benefits of a 5.1 are the descreet surround channels which will add to the "airyness" of your tweeters. A subwoofer that's equal or better then your speakers can only improve the experience. In short, if you add a center and surrounds that match your mains and get a comparable sub then you should like the 5.1 experience. If only spend as much on a 5.1 system as your current mains sell for then you might be disappointed.
A good 2.0 or 2.1 can be very enjoyable from the sweet spot if you can crank the volume up to fill the room. I can't turn the volume up that high so an inferior 5.1 (if you can call Polk LSi25's, 7's and an LSiC "inferior" - I don't) is an acceptable compromise. Low volume surround sound vs high volume "airyness".