Nope. By RAW, you get extra attacks at +6, +11, and +16, not every 5. A +21 doesn't give an extra attack, but that doesn't stop me from giving them a special ability that gives an extra attack (did the math and Figher gets and Vanguard Speed at BAB +21 ). I figured that the biggest problems with a full attack is that your target must remain stationary to do it. This, at least, gets around that and actively punishes the target further for staying still.
That sounds good, but do you think that would be too much?