The Flushmate is basically a storage and delivery system for the water and with a discharge flow around 70 GPM, any inconsistencies in the china bowl can cause excess turbulence, splashing, roll-back, and other such problems often perceived as Flushmate performance issues.
The splashing you describe typically occurs when a jet-hole or rim-wash hole is partially or fully obstructed and doesn't have anything to do with the line pressure or the internal pressure of the Flushmate. These obstructions can be caused by heavy mineral build up, a small piece of porcelain that may have gotten lodged near the opening, excess glaze, or any combination of these factors. You may be able to visually identify the problem-area by observing the action in the bowl as it's flushed.
The Flushmate's pressure is controlled by a factory-set regulator in the lower supply group and as such, as long as the line pressure is meets minimum requirements (20 PSI for the 503 series), the internal pressure will be constant. Attempting to eliminate the bowl splash by partially closing the water-supply valve will not correct the splashing bowl problem and may prevent the Flushmate from performing as it should. It may cause the vacuum breaker in the upper supply to expel water into the china holding tank which is a symptom of impeded flow or low/marginal line pressure.
Since it sounds like a recent installation, it's unlikely the problem is mineral build-up and is a direct result of an obstruction in the jet or rim outlets. I suggest you contact the place of purchase or the fixture manufacturer. Their initial response will probably be to tell you to contact Flushmate so you may need to help them understand that the problem is with the bowl and not the Flushmate.
Good luck.