When exploring your stroke at higher tempos, it's natural for your SPL to tend to increase a bit. What's going on? Generally speaking we tend to flatten out, which causes the stroke count to go up.
I find the first best thing to do is NOT to time the hand entry to the beeps on the Tempo Trainer but instead to time the beeps to the weight shift. This helps keep the whole body in sync with the stroke.
Just focussing on hand entry with beep can cause the hands to get ahead of the stroke and eventually the stroke will break down.
You will also find that by focusing on weight shift it's easier to maintain the SPL to within a stroke or two.
Try it and let me know how you get on.
I am a certified level 3.0 Total Immersion Swim Coach.