Why is it slower now New guy needs Help...

1992 LX Hatch 110,000 kms 5.0 5 speed loaded no power windows 273 gears bone stock.
The car went 15.1 @96.8 mph bone stock.

Then i made up my own ram air with a K&N filter car went 14.90 @ 98.6 mph.

Then i added 355's new posi,Hoosier 27.5x8 tires from a buddy who a stock car he races(free)hooks hard the car went 14.36 @ 100 mph

Then i removed the air conditioning compressor,lines,condesor and the smog pump and front sway bar and the car went 13.99 @ 102.5 mph

Then i put on a set of mac equal length 1 5/8 headers with a BBK 2.5 Inch h-pipe and Dynomax 2.5 inch Ultra flo mufflers dumped at the axle and the car went slower 14.1 @99.2 mph

This was all done with a G tech Pro and on the same road that i always test on (out of town)
What i want know is why would the car go slower with Mac headers and removing the cats and factory mufflers and tail pipes,i should have picked up 20 hp.Is it maybe to lean now.
Any help would be great,i think im going to just put a Zex Nitrous system on it,i still have to put my Kenny Brown subframe connecters on that sitting in my garage collecting dust and i need a shifter.

Thanks.....a mid 13 would be nice and a 12.50 on the Nitrous would be great.

Help help help...........