increasing heap dump per user from 12K to 64K would increase memory usage for 500 concurrent users from 6GB to 32GB. So the DBA should be very careful changing these values.
While I would agree that having to allocate somewhere between 6GB and 32GB of memory for user heap space should not be done lightly, I can't tell if you have a typo in your post or if you made a mistake in your calculations so fwiw ...
Increasing per user heap memory from 12K to 64K, for 500 concurrent users, would increase total memory usage from 6MB to 32MB ... so impact on overall dataserver resource usage should be (relatively) small. 
NOTE: Not trying to nit-pick your post but rather trying to make sure you're not worried about having to allocate GBs of memory for user heap space due to a miscalculation.