I recall hearing it's something like 50, with every reclass adding 10 permanent levels to the "internal" level.
The Cumulative Level caps at 20 for Normal, 30 for Hard, and 50 for Lunatic. This combines with your Current/Internal Level. So, at most [at Cumulative Cap]:
As a Level 20 Unit, you'll leve as though you are 40 [Normal], 50 [Hard] and 70 [Lunatic]. But you'll always still be able to gain levels.
Is it worthwhile getting Donny to 15 for Underdog?
I'd say yes. Once he gets past the first few levels he levels fast anyway. It should only take a battle or two to get him to Level 15.