IB's unbundled commission structure is cheap, but I wouldn't recommend them for HFT. HFT, as defined by the industry, is basically very heavy on the order flow with the majority of those orders going un-executed. IB charges an insane cancel fee for orders that are direct routed. If you let IB route the order with their SMART algorithm then you have no idea where it might land. I'd speculate that IB charges such a crazy cancel fee because they don't want high volume limit order traders competing with their Timber Hill market making outfit.

Agree with everything else you said re: time frame, etc.