FRS/GMRS = UHF
Cobra =/= Motorola
1 Watt radio's can suffer in the woods as well; and easily get their range cut down to less than 500m pending the topography. That will play into the transceiver a lot, as well as the antenna's integrity.
The point is that you have to research the radio; and not get hung up on FRS/GMRS vs. UHF.
Also, the limit for transmit power on GMRS in Canada is 2 Watt; having 5Watt could potentially land you in hot water (But not likely).
If you really want to get into increasing range, the output is not the place to start; a repeater station is! A fair number of programmable Radios could be programmed to operate as a repeater as well.
A Garmin 650 is a pre-programmed FRS radio; but by your previous statement ((as it is)) Fox, it's not better than a "UHF Radio". Yet it has 5Watt transmit capability for US Model and 2Watt for Canadian market. That's an "FRS Radio" by this "standard" that has been set. It's just annoying wrong way to look at/for radios.
Also, GMRS is limited to 2watt transmission in Canada before you start getting into licenses and permits etc. Transmitting at 5Watts likely
won't catch much attention but if it did...
If you really want to boost range, stop looking for big transmission power boosts and start looking into repeater stations. This is where "programmable" can really shine as depending on the unit very often such units can be programmed to act as a repeater.