If we flash to the new firmware are there any gains? Will it void the lifetime guarantee if we do flash it with stock 1.2.5?
At least under US law, it is illegal to void warranties unless you actually do damage to the device.
Whether you're confident in your ability to modify firmware without damaging it or not, is up to you.
Also note that overclocking (I'm not sure how to define this with BFL ASICs!) generally
always does damage, even when it works.
I guess we're talking Magnuson-Moss act...
http://en.wikipedia.org/wiki/Magnuson%E2%80%93Moss_Warranty_Actand the "Warrantors cannot require that only branded parts be used with the product in order to retain the warranty" provision.
However, we're not quite talking about a "necessary" service part like an air filter or a belt, but some unnecessary change, like a hot camshaft.
But, since BFL has many times said that the process the ASIC was made on was good for 1 Ghz plus, and that the chips were originally specced for 8 or 9GHs, one could claim that the implied warranty is that you're not overclocking anything to bump them up a bit. Especially from jally speed to speed of chips chipped in singles.
and on the other hand the warranty given "This unit’s system board has a lifetime warranty from manufacture defect or component failure." implies that fans and PSUs are not covered.
Anyway, when all is said and done, it would not be very unusual for any digital equipment manufacturer to deny warranty based on overclocking or firmware modification. A manufacturer can make convincing arguments in court for denying claim, especially when it has not offered a "Full" warranty.
That is though, only the Federal situation, some states may have warranty statutes that differ wildly.
So this is all IMO and IANAL and YMMV and AFAIK this is MO of FTC and might be SNAFU, FUBAR, GTFO or LMFAO