The solo has parameters specifying a minimum of 5 sats and a minimum HDOP of 2.5. There are other factors that go into deciding the lock is ok though,.
Is 3M 3490 the material 3DR uses for their V2 shield? I've had it since the day it was released for purchase by 3DR and it is hasn't degraded at all. I have a solid lock with 8-10 sats in less than 30 seconds every time without fail, even on the crappy stock GPS. It's the only reason I haven't bought the mRo yet.
Shielding more of the housing is probably helping you a lot. Most of the RFI that screws with the GPS comes from the camera. Other action cameras do the same thing. When I turn my mobius on, I can watch the GPS start to wander. If you shield below and around the GPS, that should be all you need to do. And that is more than any 3DR shield provides.
I'm gonna guess those parameters requiring 5 sats and a minimum hdop of 2.5 can be changed just as they can be changed in Arducopter firmware via Missionplanner. Typically for my DIY hex with an M8N on it with Pixhawk and Arducopter I set minimum sats to 8 and minimum hdop to 1.5. On a good day with that rig I usually get between 20 and 22 sats and I've had my HDOP down to 0.8.