I am new to the forum as well but had the same thing happen when I added 35" Mud BFG's. I lost a 10th of a mile each mile. I have been running 35" BFG's for 173,000 miles and never changed the odometer so have to add that .1 back in.You may not have lost as much as you think. Bigger tires mean your speedometer is slower, which makes the odometer incorrect. Therefore less miles rack up per tank.
My tire change resulted in 3 miles per hundred miles difference. So every tank I have to add 10-12 miles to the calculation... It makes me feel a little better....
I too have the cool whoosing sound after installing a Tymar but mine's louder the more I get into it. Once I hit fourth it goes Plee-e-e-sh. I hope thats the wastegate opening up and not surge.I still don't understand the pressure surge. I just put on a Air raid intake and I can hear the a whooshing sound at low rpms but into the throttle. is this the surge? And what is actually happening on a pressure surge?