Yup, I snagged a used Donaldson set-up was on a FX691,  super cheap, but looks like new (arrived today, better than the pics). 
No I'm not commercial mowing outfit, just my place, & I know it's overkill. But agree "we do overkill best". 
I have maybe 2 more mows left, & should be done for this season. I gotta get canister washed up internally & thorough inspection, & plot what I wanna do. 
Sidenote: What I find interesting is the 2 barrel FX engines use 2 different size main jets, & they have a pilot jet which my single barrel FR does not. 
Main Jet No.
FR main jet =  0 ∼ ∼ 1 000 m (0 ∼ ∼ 3 000 ft)= #124
High Altitude Main Jet then= # 122 & leanest = #120 
FX main jets=  0 ∼ ∼ 1 000 m (0 ∼ ∼ 3 000 ft)
Left= #110 &  R= # 114