i wouldn't change boost gauges , the 50 is what you need , you should be seeing 40 psi minimum , i would think 45 is a good number with those mods , i need to get another adjustable elbow , let a memeber use it on his truck and haven't seen him since , that was a year ago ...
10 psi is not alot of boost , try 5 , but it sounds like the turbo is spinning up to fast , i would see about getting a 14 housing , slow down the buildup of boost a bit , should help keep traction , your 4. 10s are also contributing to the traction problem ...
speaking of mileage , i got 19. 4 with dd3's alone on the trip to maryland doing 75mph and doing alot of playing , on the way back , doing about 70 , alot less playing i got over 20 , thats pretty close to my mileage with 275's so i think the DD3's will stay . .