The 910 is actually 20.5 HP. The 22 number was under the old rating system. The MOD isn't an option on a 910. You'd need to step up to the 920 to get a MOD, and only the 48" MOD is available on the 920. I believe the 910 with the 48" deck is fine for normal cutting, but it will starrt to lack power if the grass is wet and or thick and there are any hills involved at all.
A lot of guys on here have bought them and replaced the throttle plate with one from a 920 which is the only true difference between those two engines. It's about a $40 part and an hour or so of your time to do. There's a thread in the mechanic & repair forum if you do a search on it there.