If it costs you a grand to tint the windows, I'd say you are getting a royal screwing.
The Kubota ac system, in general, is marginal under the best conditions (good air flow through the condenser and evaporator, clean intake filter and clean heat exchangers), nothing new there, been like that for years. Kubota decided to move the ac components under the seat from on top of the cabin space and made it even harder to service. To get maximum output you need to have maximum air flow through the condenser in the engine bay and the only way to achieve that is the addition of an external 12 volt cooling fan, which most owners add.
Airflow through the various heat exchangers in the engine bay, is marginal when the motor is not at rated rpm. The cooling fan has a lot of work to do, all the time. Like I stated, keeping the components clean and the addition of an electric fan really helps as well as keeping the radiator clean and free from dust, chaff and debris. I clean mine daily when I'm farming.
...and the intake filters need to be kept clean as well as the cabin heat exchangers. Any lax maintenance on any of the ac components equals poorer cooling.