The Nano downclocks based on two things: wattage and temperature. Power Limit simply raises the artificially imposed AMD wattage limit. Undervolting in turn lowers wattage and thus temperatures. Slight Undervolting is actually healthy for a GPU and is the opposite path of aggressive overclocking in which you Overvolt. If you get stability issues then you have undervolted too far and then you simply back off with no damage done.
HardOCP found that their Nano at stock everything averaged only 913MHz. Simply raising the Power Limit to +50% increased the average to 963MHz. That was tackling AMD's arbitrary wattage limit. To tackle the temperature problem they set the fan to 100%, which is utterly obnoxious. But it gave it the full 1000MHz average:
http://hardocp.com/article/2015/12/...orm_factor_overclocking_review/2#.Vrp6LtAhGFk
An alternative and more audibly reasonable path to tackle the temperature problem is to undervolt. I'd suggest starting with -24mv, and trying -48mv if the former is stable but not giving the full 1000mhz. FYI you can only change voltage in multiples of .06mv. You may need a custom fan curve as well, but certainly you won't need the full 100%.
I think tweaking the Nano would be very fun. But at the very least giving +50% power limit is an easy solution to increase performance and doesn't even need MSI Afterburner. If you don't want any babysitting on it though I'd think a factory overclocked Fury may be a bit faster and also likely quieter, e.g.
1050Mhz Fury, but it asks for more money.