Burns more gas.
Umm
Have extra maintenance on the prop over a fixed.
Umm...
Climbs faster? hehe
A 180 Constant speed on a 7GCBC was not factory approved. Make sure the paper work looks good, such that he wont run into a snag down the road. One of a few reasons we stayed with a fixed pitch prop on the High Country 7GCBC was that it was already cruising at 20 mph under Vne at 75%.
(other reasons, weight, cost, too much 8GCBC overlap)