I have not had the pleasure of driving a 149 yet (need to play around at Lew's house more often hahaha) so I can't say for sure how fast they go. I was pretty surprised how fast my 127 drove since before that I really have only drove my 108 and 73 which are both gear drive. Since the 127 really motored down the street I thought I would ask the same thing you are now
I don't know enough about what the PO could have mod'd on your tractor. I guess they could have messed with the gears in the trans but I would be surprised. If the engine is not rev'in like crazy I guess that's just the speed. All this being said there are lots of folks here on the forum with 149's and other tractors so they can give you a better idea how much faster it runs. A 149 is on my list of toys to get so hopefully one day I can tell you first hand