I play Eldar, but I can still say that Pirhannas (and Eldar Vypers) can do a lot to slow him down significantly.
if you turbo boost a skimmer in front of his Battle Wagons, he won't be able to get a 2nd turn charge. If you dodge his ram (3+ on skimmers) his deff rolla doesn't go off. And his wagon stops dead in its tracks.
Sure his lootas will probably blow your skimmer sky high, but shooting phase comes after the movement phase, so he'll either have to go all the way around your skimmer, or try to ram through it in which case he has a 2/3 chance of not moving at all.
Basically you are sacrificing an 80 point skimmer to hold up over 400+ points of ork phalanx for a turn. If you sacrifice your Pirhannas one at a time, you can probably turn a 2nd turn charge into a 4th turn charge, and have your orc opponent frustrated beyond belief

!! The more turns hes slowed up, the more turns hes eating long range fire power!