Nearly every attachment of similar size will perform better on a pto driven tractor which has much lower engine hp. A pto driven diesel engine will develop more torque and hp as the rpm of the engine slows down from recommended pto speed. A hydraulic motor has max hp at max flow. Any drop in flow as the attachments gets more work/resistance, immediately results in a drop in hydraulic motor pto. This is why a direct drive/pto CUT with 30 pto will out perform the some hydraulic motor with same pto hp. It would take flow rates in the 40-50 gpm to make me happy with the power characteristics of skid steer attachments to equal my CUTs pto.
I too own and operate several hydraulic driven attachments including a rotary cutter, trencher, tiller, soil conditioner, concrete mixer and stump grinder, powered by medium frame skidsteers with 15 to 17 gpm and a high flow on the 15 that bumps it to 24.
Pressures range from 2600 to 3300. Typically a bit more pressure then most tractors.
There is no doubt a shaft driven pto will transfer all of the engine power. But thats not a option for a blower on a fel. So if the goal is to blow snow going forward with an attachment on his fel then there are some design limitations he has to live with. Reduced hp is going to be one of them. How much it gets reduced will depend on the previous specs mentioned and how he designs his system.
Is this going to be enough hp for him?? I don't know,the only way to answer that is for him to test a hydraulic blower to see if the loss in speed is worth not having a kink in your neck.
Until you use a hydraulic blower, it is like try to explain to someone that has never driven a car the difference between chevette and a corvette in terms of hp numbers. They are going to have a hard time knowing how much they will be satisfied with.
Personally I'd could live with the 15 gpm pump if it let me work in forward. Especially if he has hst, if it begines to stall you can slow down a bit without wearing the clutch. I love hydraulic drives, they are very user friendly and easy on parts. Is it going to out pace a pto blower, hard to say, but some of the lost is blower performance will be made up by tractor performance resulting from driving forward.
Ken