I can't give you raw data like % of increase in blade tip speed, HP required to drive 4" vs 5", nor can I give you personal experience from the upgrade. What I CAN give you is plenty of experience with the 5" pulley on the 44A deck. In my opinion (and based off of what I've read by guys that have done it), I'd say it would be beneficial if you cut wet, thick, or tall grass even somewhat regularly. At my current house, I used the 44A deck when I first moved in. For most of the yard, it did just fine; no complaints. But I have a couple areas that grow faster, thicker, and taller than the rest and in those areas, I would have liked a bit more from the deck (being driven by both a KT17 S1 & M18 so HP wasn't the issue) If you keep your yard trim, don't mow wet or anything like that, I wouldn't think there would be a need to upgrade.
That being said, I'm sure others will be along shortly to give some real world experience aka actual help
