You void your warranty. But given that the MF79U costs less than $40 used on eBay, the risk is minimal compared to the performance reward.
A: That band doesn't reach your house. Reset the modem (paperclip hole) or use AT+ZRESET to revert to auto-band.
: Modded firmwares sometimes optimize power consumption, reducing the heat generated during heavy data transfers, which prevents the device from thermal throttling.