The HH is slightly lighter in weight and seems to have a flatter neck radius as well.
P90's would be interesting, i have a Gibson LP Junior so i am a fan of them pickups but i do beleive a lot of that 'killer tone' comes from the set neck and slab of mahogany..... something the Jag does not have.
As for the conversion not being popular i think it reflects that the Jaguar is'nt a 'common' guitar and its users generally opt for the classic Jag with tremelo. Of course you have a great point about the TOM already being on, and there is also no need for any routing whether you use humbuckers, P90's or single coils.
It seems like a winner for the non-traditionalist or the Jag lover looking for a 'rockier' guitar.