The whole difference in tension is a highly debated issue. For years I strung the gut mains & poly crosses at the same tension (54lbs). Then I read here that people were suggesting the poly be dropped 5-10%, mostly due to the realization that poly plays better at lower tensions.
So, I dropped it 2lbs about three years ago when I changed frames, and have continued doing it even since. I honestly couldn't tell you if it made any difference, other than maybe a slightly softer string bed.
The only way to truly determine any difference is a 'blind taste test', where you string two identical frames with the exact same string and drop the tension of one. Then, not knowing which is which, go out & play a couple matches with each and try to determine afterwards if there is any difference between the two.
"Nobody beats Vitas Gerulaitis 17 times in a row" -- Vitas Gerulaitis, after beating Jimmy Connors on his 17th try.