The max flow is what that pump will put out without anything hooked up to it. You are currently running your pump very close to the rated performance...that's a good place to be. Moving the diffusers deeper will cause it to dip below the Rated Performance and add stress to the pump which will likely shorten it's life. How much, who knows. I'd leave them at the 5 foot depths, but you could move them elsewhere in the pond if you'd like, just keep them in 5 foot of water or a bit shallower. I have found that my diffusers can sit 3 foot off the bottom (on home-made stands) in 7 foot of water and still circulate the 10 foot deep water that is 40 to 50 foot away. Your diffusers have a good chance of pulling water up from the deepest part of your pond while sitting in the 5 foot depths. So long as you tested temps at all depths and found that minor temp difference, I'd say your system is turning the water over well. My only concern is that it is early in the year and ground temps and air temps in your area may be equal (on average) and this may be giving you a false sense of security. eg. The ground temps are 55° F and the average air temps are 55° F (or close)...whos to say of the water IS getting turned over and not just staying consistent because of the surrounding earth and air temps ??? The temp testing needs to be periodically performed as summer air temps increase. When you turn off your aeration, you are looking for cooler bottom temps and warmer temps towards the surface. The bigger the difference between bottom to near (~18") top (without air), the more confidence you can put into the temp testing once the air is turned back on. You may find little to no temp difference without aeration this time of the year and have to wait until it warms up a fair amount.

There is always dissolved oxygen testing that you can do, but I have not mastered that at my pond and cannot offer much assistance other than how to get the water samples. Even with aeration at my pond, the DO seems to differ from top to bottom in the water column and does not seem to be as reliable as temp testing. We will have to rely on some other PB members to help determine what DO levels are considered good from top to bottom.

EDIT: And, without Lift-rates for the diffusers, you will have to rely on testing at the pond to know for sure. The diffuser specs sure seem to mimic what I have seen for other 9" Matala's and are close to the Vertex brand that I am familiar with.