Lon,
I always thought the reason for the 330 cap was because the high level sector was too busy. Most people don't put traffic ex SS above 330 just so they don't have to co-ordinate but there are many departures ex LL/KK that could make 340+ and so no phone call necessary. Often that would aid things at our end-after all if they are parallel at the lower levels we still have to make a call so not saving anything on your part.
Just tell us no co-ordinations allowed from lower into upper rather than a level cap and we'll sort it.