The cobalts go dull quickly,have tried them drilling out drilling out broken exhaust manifold bolts out of cylinder heads.
I don't mean the following in the rude, snarky way that it's usually used, but you're doing it wrong if that is the case (or the bits are doing it wrong).
Generally, as far as durability goes, Cobalt falls directly in between HSS and Solid Carbide/Carbide tipped.
I think the problem you're running into in this particular case, is that the manifold bolts are heat treated, so it's essentially trying to drill hardened steel.
Although Cobalt bits are more durable than HSS, either will dull very quickly when drilling hardened or heat treated steels.
Your best bet for this application is probably Carbide tipped, as solid Carbide can get rather expensive if you don't get a killer deal on them (such as ebay).