Every array has engineering tradeoffs - some have improved throughput for certain activities at the downside to others. Its all very workload dependent.
In terms of a performance 'knee' around capacity utilization, you are likely referring to a charge thats often levelled at NetApp and WAFL. NetApp will claim it doesn't exist. Every competitor will claim it does. The truth is likely somewhere in between. Some people might have hit one, some might not have.
All the arrays on your (with the possible exception of the XIO system) are very strong systems, and, if designed right, can meet your needs.
Don't believe the FUD throwing.
(disclaimer: i work for emc, so obviously a competitor to netapp).