Personally I'm using 5 acceleration right now because using anything less, the overall aim of the game feels too slow for me, no matter what settings I put on the XIM, and worse when at 1 accel, I can really feel when it kicks in, and is rather jarring.
It's kind of like turbo lag in a car. Really struggles to move at low RPM but when the turbo kicks in, it's a drastic change in output. That's what it feels like to me with 1 vs 5. I assume it's because with 5, the accel kicks in so quickly/early that I don't necessarily feel the "jolt" of it, and I can more easily adapt to it. If that makes sense.