Mk I Deb turret - 700m/s
Mk II Deb turret - probably 600m/s, not sure about that
Anyway, most guns are 600 or 700 speed, and 50m/s isnt that big difference. I still recommend turning Debilitators off when the shield is down. Conserves power and improves aim slightly.
If my guns are in the middle of the 750/650 issue, I usually take the slower ones, so the targeting computer compensates with more lead instead of less lead. I feel it usually makes me miss from behind, not from in front, so anything helps.