I've got a Sting and recently installed a Fralin '51 P bass split coil. To cut to the chase, I feel this could be the single biggest upgrade any '51 P reissue, Sting or Squier CV50's P owner can do. This is now my quietest bass; the first time I plugged it in I thought I had mis-wired it because there wasn't a trace of hum or buzz. I started playing and broke into a grin; the tone is rich & full with a clear top end, I'd swear it it's a single-coil.
I bought my Sting second-hand and it came with a Duncan SCPB2, sounded good. I was curious about how other pickups would sound so I then put in a Lollar single-coil, also sounded nice, a bit lower output, perhaps more polite than the Duncan. But after getting used to the low noise floor of DiMarzio Ultra J's in another bass, I didn't want to put up with single-coil noise anymore.
With the Fralin & roundwounds on my Sting, the tone is pretty much what I'd hoped for, though I do cut the highs about 4-5 dB. And ideally I'd like the mids to be more forward. I called Zack at Fralin and he said to send it in for a 5% overwind, says it should accomplish both goals. There is no charge for this, I'm very impressed with that level of service.
Somebody else here already wrote up a favorable review of the Fralin '51 split pickup and he pointed out two shortcomings which I also found to be true: the bottom bobbin will not fit into the standard size pickup cavity without being filed down a bit. I told this to Zack and he said they'd check into it straight away. Two minutes with the the Dremel took care of the bobbin. The second thing (and this is very minor) is that the leads are too short, but I just soldered on 5" wire extensions and problem solved.
I wasn't going to post until I'd completed the journey & received the overwound version, but thought somebody else sitting on the fence might find this info useful. Again, I don't want to imply that the top end is at all harsh, but if overwinding the pickup will push the mids more forward and darken the top end just a bit, I figure why not. If you're using flats, you might prefer the stock winding.