Both 1040 and skyline are great pipes for the 3 port OS.

The 1040 has a little more torque, the skyline a little more top, I tried and used both.

Stick with the 5.2mm for the 1040, the 6 (and 7) mm is for the 5 port outlaw engine. I heard it runs very bad with the 3 ports engine. Lacks all the great torque. Never tried myself.

Fuel economy is great with the 1040... got 2 extra minutes compared to a mega ZX with mega stock pipe. Speed is a little bit lower, torque is much higher.