It's my understanding (not trained on the specifics, just on how to install it as a contractor) that bonded service syncs at 17 meg per pair for 35 Mbps service, and the TV & Internet are not separated. Bonded service was for customers who are at about 950 metres and above, from the DSLAM. I haven't seen Lightning 50 in service, but I assume they are just upping the sync rate per line (25 Mbps at least per line) So again, the who service is just split on two lines and recombined at the modem.
As far as inconsistency, you could check the sync on both lines in the modem interface for major errors or loss of sync (not sure i should mention it here even though its easy to figure it out haha. Message me if you need it) The upload sync rate may not be affected by certain line conditions?
if it's not a bad line, it could be the speed test. Try the MTS ftp site (ftp.mts.net) and do a download of the test.exe file and watch the transfer rate as it's downloading. It's a dummy file, so you don't need to complete the download. Keep in mind that it will show Megabytes per second, not Megabits. I forget the conversion. Good luck!
Authorized Service Provider (Contractor) for MTS Allstream. Any views expressed are my own.