Will it be slower? Depends. If it's USB 1.x then it can cost you a noticable framerate hit. This was the case with the first Creative Labs USB sound cards. Since those came out well after DSP-300s, I'd check Plantronics website... pretty sure the DSP-500s are USB 2.0. Wish I still had my Counter-Strike Edition set

If it's USB 2.0 you probably won't notice any framerate hit at all. Audiophiles will notice the difference in sound quality between the DSP headsets & an audigy 2 or better (I can't, my hearing isn't the best...).
In other words, it's very likely they're gonna work just fine.