I was thinking about the elite ship xp problem too. I would say, that the best thing to do is enabling conversion of elite ship xp to another in-game resource at reasonable exchange ratio.
Conversion examples:
4 ship xp -> 1 captain xp (same overall boost as tank with crew of 4 in WoT)
1 ship xp -> 10 credits
10 ship xp -> 1 free xp (worse than paying, but still some use)
Good thing about conversions is, that you can adjust ratios to prevent "breaking the game".