considering this car is pretty laggy down low and short on torque some of what you may be experiencing (albiet weird) could be normal. I was gonna suggest plugs as a starting point to check but w/ 25k I don't think those are culprit. Fuel Filter? Again, 25k on the car I wouldn't worry.
You mentioned the Akra Evo torque dip which I don't get why this is the most popular/expensive exhaust and it has a drop in power?? And torque of all things? When viewed on a dyno plot its def quite apparent.. I have never driven one w/ Akra so can't comment on that but after seeing it plotted on a graph I could def see that being felt while lugging the car around