Thanks for the feedback folks.
I'm working on merging the 374_2078 GPL code into the main tree, which should hopefully also take care of the USB issues (I haven't finished merging yet, so I can't tell at this point if it will take care of that issue or not). No idea when that will be done, will probably a couple of days as I will need to test it for a bit.
On the N66U driver front, I was able to do a few tests. Strength-wise, the signal doesn't seem to be at the same level of the SDK5 driver, tho it did seem to have a slightly stronger RSSI. I suspect that most of the changes in this new version were in regard to PPPoE acceleration, and possibly general stability and performance - I can't tell for sure as I never had any problem with the previous driver. There's a thing or two I want to test once I get a merged code tree however.
The oldusb builds were showing some other issues here when I did more tests with them. My RT-N66U would get stuck whenever I tried to reboot it for instance, requiring a power cycle. Therefore I will pull those oldusb builds from the download site for now.