With the change to 16+ boost on the Spit V it is not at a power disadvantage anymore. When it had 12+ boost the IX had an advantage in the verticle, but no more, and the IX is only 2-3 MPH faster on the deck now, where it was 20MPH faster.
The only advantages the IX has over the V now is hi alt performance, the .50 cal gun loadout and the ability to take DT's. Otherwise the V owns it IMO.