It wouldn't necessarily be too much but it depends on the model and wattage of MVB. As far as too much, they can handle quite a bit of UVB in bursts but since we have them in relatively small spaces it's best (in my opinion) to offer a happy medium of about 3-5 UVI in the basking area. They can take more than that in the wild but can also burrow and hide easily. For that reason, it's also important to create a gradient so there is intense UVB available on the basking area but little to no UVB in other areas in case they want to get away (which they often do).
A T8 with a reflector will produce about 2.5-3 UVI at 6'' when it's new. A MVB can create quite a bit more than that, but they vary a lot between brands and wattages. The downside of using an MVB in a main tank is that the center can be quite intense and then drop off quickly as you move away from the middle of the beam. The beam is narrow so it doesn't allow for even basking the way a longer tube would (wide, even beam of UVB vs a cone). You also need to set a MVB according to it's UVB output (within a range) and can't adjust it's heat output independently. For this reason I prefer using MVBs as portable lights and travel lights while I use tube UVB lights and non UVB emitting basking lights in my main tank.