A 3000 AP that has a mixture of AC clients and legacy N clients (F200, etc) will speak MU-MIMO to AC clients, and normally to legacy clients. There's an estimated 10% overall performance reduction while running in this mixed mode. Legacy clients will possibly see some signal improvements due to diversity gains from the 4x4 antenna.
In terms of overall performance:
A 3000 AP with only legacy clients should perform about the same as a 2000 AP (-10 % hit for legacy/mixed mode)
Next best performance will be a 3000 AP, with a mix of legacy and AC clients. (MU-MIMO, AC up to 256QAM, -10 % for legacy/mixed mode)
Highest performance, will be a 3000 AP with only AC clients, no mixed mode. (MU-MIMO, AC up to 256QAM). In tests we've seen 200+mbps using a 20MHz channel with a decent mix of AC clients spread out in such a way they can trigger MU-MIMO.