I know for a fact that lack of backpressure on a stock car(low end stock car) tends to bog. Mine does, its a civic. Maybe not in a BMW or Mercedes that have much more ECU involvement at any givin time then my EG civic, which doesn't do but so much other then a few preset commands. I doubt the sentra in question's ECU is that up to the challenge of fuel tuning stock. Or a stock manifold being used wouldn't be considered a well designed header, do you consider it a header?
Not tryin to bash, but even turbo cars lose power from lack of backpressure with a very nice free flowing exhaust. Its usually a 1 hp, or ft/lb loss off idle, but makes up for it much more up top.
Look up in some magazines at downpipe installs with a catback, very little power is lost due to lack of backpressure, but the upper RPM results is worth that 1 or 2 ft/lb