Hi,
I didn't made any tests, but I guess that the relative maximum for intensity will not change if you use higher power limits. The difficulty comes from the pool. Since the MTP solutions size if more than 200k I prefer higher diffs - so you can save a lot of bandwidth for 'important' traffic on your line. Sending a single solution per minute is fine from my point of view - but there are surly other positions out there.
I didn't change the power limit all of them are set to 100%. Anyway thanks for your answer about the difficulty I thought that I can manually set it to higher diff to make it stable and little hashrate improvement.
-i 20 gridsize 4096 normal works
-i 21 gridsize 8192 & not not enough virtual memory

I'm not getting this error when increasing the intensity I'm using awesomeminer but I don't see the error the same as the image.
-i 21 is honestly not working to me -i 20 is working fine to me same as the intensity 19. Do you know some command for reducing gridsize without using the intensity?