Who knows if a Mischblut only has 50% of a pure blood quincy, 80% and more, or less? It could be very random. I don't think that Quilge would have mentioned a far stronger potential of Uryu's attack, if their potential would be capped by half just because one parent wasn't pure.
ah, well, i think that he will be SWP and maybe his letter will be "L" for light, also the way he is refered to in the whole arc makes it almost clear he will be very important in the vandenreich regardless of not being pureblood, and as Schabrak stated, if the power has anything to do with genetics or has a somewhat similar way of being propagated, then he may very well have all the power of a pureblood or almost all of it.
lastly i think that the bow he has been using since the arrancar arc will be his spirit weapon no doubt, only he didnt know all it could do yet.
and here is one last observation to back my previous claim: when you look at all the quincy in the vandenreich it seems not all have speccial spirit weapons, only the ones who seemed to have some importance had them, remember when ichigo fought all those quincy in hueco mundo? most of them had only a normal quincy bow, quilge was the only one with a developed spirit weapon. this combined with how uryuu got his spirit weapon upgrades (by using the sanrei glove and letz still), leads me to think that only the quincy with volstandig have speccial spirit weapons, wich leads me to wonder about the power of those arrancar, they said they where valuable soldiers, one of them said to the other "do you know who you are talking to?" and the one talked down to had a speccial spirit weapon, this seems to indicate that the arrancar soldiers are actually very strong, maybe not sternriter level but close, this really seems to justify them being called precious.