In terms of how your tractor performs, i.e. how much work it does and how fast it does it, your
engine output is your primary limiting factor.
In an HST tractor, the power that is transmitted to the wheels to do work is a product of the
HST's flow rate and pressure. So a 25 hp engine might deliver, say 21 hp to the wheels thru
an HST, and maybe 23 hp to the wheels thru a gear trans. The HST is less efficient.
On my Kioti, the HST uses 2 pressure relief valves, and they are not adjustable (without surgery).
The CK35 shares one RV..... I don't know what the 2510 uses. Setting the HST RV pressure
higher will not increase the work output of the tractor, however.