PWM'ing FET's works great, specially at high PWM rates, but say for example you are getting 6 amps out of a FET on full, PWM'ing the 6A means you are pulsing 6A constantly. The way the 7135 works is it limits output to 350 mA, so to get 350 mA out, just drive it at it's fill capacity. So instead of continuously spiking 6A out, you get a smooth consistent 350 mA of output. This works way more efficiently than PWM'in a FET to get an equal amount of light output.
So we use FET+1 circuits/drivers for this reason. If you PWM the 7135, you get very efficient low modes, all the way from less than 1 lumen up to about 150 lumens.
Take that one step further like HQ did on this board design, add in a bank of 7135's and you get 3 output channels: 1 7135, bank of 7135's, and a FET. This way you get efficiency out of the middle modes as well, using the bank of 7135's. Wiring 7135's in parallel effectively adds them for total amps output. 8 7135's is 2.8A, 16 7135's is 5.6A, etc.
Bout that FET, there's a few things to look for. DEL and a few other BLFers could analyze the specs wayyy better than I could. I don't take the chance - I only use what's been reviewed, and then proven.