Just got your email and agree with Otto keep it simple. You might be able to raise it a bit more but I would need a lot more info. Like elevations of top of dam to inlet of pipe and then outlet of pipe. Would need to know if you have a air vent welded on pipe to prevent swirl.
Keep in mind it takes a 24" riser to load a 12" pipe when water is about 3" above top of riser, a 30" riser will load it quicker. These larger risers must be anchored with concrete or something due to buoyancy. And also keep in mind that you are fixing to put more water pressure around your pipe than ever which is not a problem if you installed good anti seep collars. If you decide to bring level up to within a foot of emergency spill weld a 4" collar on bottom of riser and screw a 4" plastic cap in it. This will allow you to drain pond down and cut top of riser down some if need be.