Somebody dropped the ball on the Montana website the 2840 is the only one with a full set of specs in the compact tractor line. It is listed as 28 gross engine hp and 23 pto hp. The amount of drop would be very close to the same for your 30 gross engine hp and is a fairly common amount for that size tractor. 25 pto would most likely be a safe bet.
Here is some data from a test about efficiencies of engine and PTO. Of course, logic will tell you that a gear transmission will have less loss than a hydrostatic transmission.
Another interesting comparison can be made using the maximum available drawbar hp at 100 percent load and the maximum PTO hp. A ratio of the drawbar hp to the maximum PTO hp is an indicator of the efficiency of the drive train of the tractor. Consider Test Nos. 1255 and 1257 for an IH 986 diesel with 16-speed transmission and IH hydrostatic diesel 186. These tractors have identical engines, producing approximately 105 PTO hp. However, the standard transmission 986 produces 90 drawbar hp, while the hydrodrive produces 80 drawbar hp.
The comparison of drive train efficiencies is as follows. IH 986: 90/105 = 86 percent; IH 186 hydro: 80/105 = 76 percent. The tractor with the standard transmission has less loss in the drive train and is able to transmit more power to the drawbar. The overall fuel efficiency with the standard transmission is obviously better. In this case, the potential buyer would have to decide whether the advantages of the hydrostatic transmission were worth the increase in fuel consumption and efficiency.
I don't know if Kubota is overly conservative with PTO ratings, but, I find it a bit odd that when you have several models that are essentially the same tractor, but with different HP engines, that the higher HP models seem to loose more PTO power to the hydrostatic. I can see the drawbar power being a % of engine power, and thus the model with the larger engine will lose more HP getting that power to the wheels, but the PTO doesn't use the HST.
Case in point:
B7610 - engine=24HP, PTO=18HP
B7510 - engine=21HP, PTO = 16HP.
The B7610 has 3 more engine HP, but only 2 more PTO HP. The B2920, B2620, and B2310 have similar differnces (there is like 3 engine HP between each of these three models, but 2 PTO HP).
I would think that because the tractors weight about the same, the the power required to move the tractor with the HST would be about the same, regardless of available engine HP, and that the full difference in engine HP would be available at the PTO. Also, would more than this PTO HP spec be available for stationary use (such a chipper or generator), since the HST isn't being used?
Tractor engine power can be used in several ways. The most common uses are the PTO shaft and the drawbar. PTO power is easy to accurately measured, and is the most common way of describing tractor power. PTO power is about 96 to 98 percent of engine power
Anytime that gears are used to increase of decrease rpm, there will be some HP losses.
I believe the HST is always pumping fluid when the tractor is running, and fluid is moving, with very little pressure, and taking up some HP, therefore the difference between a gear tractor PTO HP, and one with HST.