Ok, I should have said it won't slow you down as much. That is, it slows down each individual some, but the total amount of experience between them all goes up.
Meanwhile if leveling is a priority, consider the 'general' profession. And maybe pick the race for your sovie that gets a 10% xp bonus.
Meanwhile, a quick calculation of champ-stacking with 'trainer' abiltiy. Assume that you take out somebody worth 50 xp. Alone you get the 50, in a stack of 5 champs each gets 10 (that is the formula, right?). Now if each of the 5 has trainer 3, each one adds 45% to eveyone's xp (10 for trainer-1, 15 for trainer-2, 20 for trainer-3). That's 225%, so everybody 22.5 xp and the stack gains a combined 112.5.
Things get more complicated if your profession is 'general' or if any of your guys have 'knowledge' or 'potential'.
Then things get crazy if you use 'paragon' to level-up your champs. With enough trainer/potential/etc, 1 cast gains 5 levels, not just one.
Also consider that the combined stack will be stronger and may get some extra xp just by winning more fights against tougher bay guys.
Anyway, late-game my sovie will be over 20, and the paragon-boosted champs will be anywhere up to about level 100.