Exhaust upgrades..How much power??

Hard to say. It depends if enough air was getting by the cracks and screwing up the O2 sensor reading making the car run richer. If its been running like crap, you'll probably gain a good amount of power. If your headers were leaking for a long time and you just never noticed a lack of performance, probably 0 - 5 hp.
What headers did you have on the car before? I'm about to do the same on mine, I'm pretty sure my old Mac's are leaking in multiple areas. I'm getting BBK's as well.
As for the catless H-pipe, probably 8-12hp at best, but the sound will make it worth it.

Yes ... speaking of performance alone

Exhaust upgrades are one of the worst mods you can make for the buck
spent ... in most cases

Of course ... if you had done a good bit of work and THEN upgraded
the system ... you would see a greater gain

but still ... it would not be all that much more

I remember a guy several years ago had a 347 Stroker with every
supporting mod in place ... EXCEPT ... he was running the oem
catback

He changed it to some kind of after market cb and re-dynoed it with
a gain of only like 10rwhp

Soooooo ... considering he was moving a good bit more air with that
Stroker than most typical stock, close to stock, or even ... h/c/i
combos ... you can see the cb is mostly a mod done for sound and
not power.

I've always felt the oem 4-cat mid pipe is the most restrective
part of the entire system

I don't know anything about your combo but
If you are stock or close to stock

I'd think those two mods ought to give you about 10rwhp

Grady