The boost will usually become problematic because bots most likely utilize it, creating even more issues with bots. However, if bots were properly removed, I don't think the boost would cause any issue. My friends and I all go back to classic because we enjoy TBC. The reason they join is that they can increase beyond vanilla content leveling.
All our characters exist in Classic already. They don't really want down time for people to go to TBC. The entire database will be copied prior to launch. If you select TBC, they will change the "active" portion of the TBC database to true and flip the "active" bit on the Classic database will be changed to false. Then you can be charged $35 to flip the Classic bit back.