How much HP gain going to dual exhaust, headers, Mustang H pipe, glass packs?

[stinkydogfilms]

1991 GM is att he muffler shop. When the motor was just replaced there was an exhaust leak. The muffler guy says to get rid of the cats, put on a Mustang H- pipe, (dual exhaust) dump into glass packs and dump out and turn down a little before the rear axle.
The motor is an 89 HO with a LOPO top intake, LOPO throttle body and EGR, stock LOPO fuel rail and injectors. Basically an HO bottom end including HO heads and HO cam.

It is now getting the exhaust work done.
1) Just wondering what the exhaust work will do in the HP increase?

I figured the HP before the exhaust work between the
1991 LOPO 0f 150HP and the
1989 HO of 225 HP

2) Are these two figures from the factory at the rear wheels?

3) maybe 180 HP before the exhaust work?

Comments?
Thank you,
Mike

[86VickyLX]

You need mustang headers in order for the Mustang H pipe to bolt up. If you have the HO cam, heads and such, you have an HO. Change the injectors, and the computer at least.
BTW, that engine must be running like complete shit running on LOPO injector firing order. Especially if the spark plug firing order is set for the LOPO...

[Neal 97 250]

I am getting ready to do this myself on my TC.
BBK 1515 headers or 15150 ceramic coated $189 - $299 your choice
Mustang H-Pipe for '86 - '93 Now on sale at Jegs for $109 !!! (That's $30 off regular)
Mustang mid pipes $41.99

[merc91]

thats at the flywheel. and i have that same exhaust and did it at the same time as ho upper intake and tb and probly picked up 50hp... with cold air intake and k&N filter, a little timing bump to 12-14* A/C and smog delete also. exhuast alone ide say you will pick up 10-15.