Lots of smoke... This sounds like FUD we used to hear 10 years ago... any file system that is full isn't likely going to perform well and no idea where they came up with the 50% number. Sounds like they don't understand NetApp and have some really old FUD that didn't hold water back in the day either. You can also reference the SPC1 benchmark sustainability test http://www.storageperformance.org/results/benchmark_results_spc1/#spc1e where the allocation is over 50% ... ~12,000GB allocated and ~3,000GB free. The majority of my customers run at over 50%...easily 70%+ allocated with some margin for growth.
NetApp also has some really great sizing tools that we have trusted for years... when we plug the inputs in and get the controller model and spindle count and type we are confident it meets the performance.
Also with your VMware, NetApp can typically give a 50% on deduplication so your allocation will be double on the other vendor....there is also a 50% guarantee from NetApp with an analysis done by professional services before implementation.