The sherman transmission is mounted in front of the single transmissioin input shaft and does affect PTO shaft speed.
I have 51 and 52 8Ns both with Sherman combination transissions. I use the 51 to open areas with a 5' cutter before sawitching off to something larger. I leave the Sherman in underdrive 90+% of the time as I find yhe reduced ground speed and increased PTO torque beneficial.
The 52 mounts a carry all and serves as my "Quad." It occassionally sees overdrive.