subaru2000

Had a 3.5" round tailpipe magnex backbox to my otherwise standard 2000my classic with 34k fitted today and it seems to have affected the boost pressure.at one point driving home accelerating in 4th gear 3000rpm the boost gauge was showing 1.13 bar which is higher than its ever been, yet accelerating hard in 2nd and it was not showing as high. also the car dosent feel as quick as before it was fitted. any advice?