I regularly mow between 6-8mph to keep production up and feel a few more mph is possible while keeping a decent to good cut as long as you aren't getting bucked off the machine. One of my mowers (old Rich convertible) was only running around 6.5mph per my GPS and have it around 8.5mph now per my GPS. In spring when grass is growing faster and thick I keep speeds down to about 6mph or till I can't hear the engine strain, but after that it's full out as often as I can and the cut is still very acceptable. If you are serious about mowing faster you have to do more than increase ground speed as others have said the cut will degrade. While I have not changed pulley sizes to achieve faster speeds (keep in mind hydro pumps are only rated for so many input RPM's and without antifoaming agents the hydro fluid can cavitate, bye, bye hydro pump) I have gone to larger diameter tires which not only give you higher speeds but also give you a cushier ride while traveling faster. For example, if you have a 20" tire OEM running 7.5mph and switch to a 22" tire, your speed should now be at roughly 8.25mph. Not a huge jump, but on a 60" deck good for an additional 1/8 acre per hour production, so an extra acre per day just by going to a slightly larger tire. Don't forget to level your deck. I make sure to check the engine RPM every couple months, those engines are rated by the manufacture at 3,600RPM under load, and governer springs weaken over time (I will also note that I've tached out over 100 machines and not a single one has ever been at 3,600, most sit around 3,200RPM-3,400RPM). This is essential for the highest blade tip speed and ground speed. No, this does not void your warranty as the engine is rated at that RPM (and yes, I have worked in the warranty department for multiple top name commercial brands). I also run nothing but high lift blades except in the fall to chop leaves, gives you more deck vacuum to keep those blades of grass standing straight up in less time to cut them evenly as you pass by faster. Now keep in mind those high lift blades sap more power from the engine since they are essentially higher pitch fans that requires more power to turn at the same speed. I will also note but will not recommend what I have done, which is install smaller pulleys on the deck for a MUCH higher blade tip speed. In my case the OEM BTS was in the ballpark of 16,000fpm and I increased this with smaller off the shelf pulleys to roughly 19,000fpm and made a cheapo MTD RZT cut and stripe as well as any commercial unit. Also those larger tires have more rotating mass and therefore more rolling resistance also using more power to run full out. So if you don't feel like swapping to a more powerful engine, I have found a combination of Fitch Fuel catalyst and iridium spark plugs is a great deterrent for bogging and really seems to help the torque output through more complete combustion. Running full synthetics everywhere, including deck spindles helps as well. Oh, and you can find K&N air filters for most small engines with enough research for better filtration and a bit more oomf too.
Food for thought, I keep hearing those newer Dixie Choppers still cut really nice at 15mph....and when doing large acre parcels, 8mph feels like you are crawling.
. Or at least call the manufacture and ask.
. I usually change tires on all my mowers anyway since I have some serious slopes to deal with, and go with ag style ATV tires to get as much sideways grip as possible, why not add a little speed and comfort while I'm at it? This is commercial equipment, and as long as you aren't thrashing your equipment in the first place a 10% increase in tire size isn't going to start breaking wheel motors...unless you are trying to tow way too much weight with it (only the Dixie Chopper Classic and Excaliber are excluded from that statement). Sometimes mowers don't go out the door with max ground speed that was originally intended as well, do a search on here for the Ferris 3100. That's a 12mph mower that leaves the factory at 10mph so the dealer can reset it if the customer desires. 
