I agree that when using a T5 you should switch the basking light out. I use a 90w PAR38 Halogen Flood Light and it works well along with my T5. I use my MVB as a portable basking light for around the house. It's tough to know for sure without using a solarmeter but there are tests that have been done that provide pretty good estimates of output at various distances.
Which MVB are you using? The 100w PowerSun is relatively weak, throwing out around UVI 3.0 or 47uW/cm2 at around 12'' which is comparable to a T8 light. The 160w is a bit more powerful at that distance with UVI of around 4.7 and uW/cm2 of around 83. These numbers are fine on their own but I wouldn't combine them with another high output bulb. With a reflector, the T5 ReptiSun 10 puts out UVI of around 7.3 and uW/cm2 of over 200 at that distance so even on it's own it's better off at around 14'' unless you can double check your output using a solarmeter. The actual output will vary depending on the fixture and reflector.