Didn't know you could stll get a high performance cam for a 2.8 v6. I would also change timing set while your in there. And it wouldn't hurt to upgrade the lifters, valve springs, & rocker arms to go with the hp cam.
On the other hand, if your talking about a new standard cam, I see no reason to change the cam unless the cam lobes are worn. With 75K on the clock your engine, if properly maintained, should be like new.
My 90 3.1 v6 has close to 300K on the clock. It still runs fine.
That 2.5" exhaust you put on the car is too large a diameter for a stock 2.8. That's why you have lost low end torque. If you modified the engine with a way bigger cam,high compression pistons, long tube headers, ported & polished heads, etc, then a bigger diameter exhaust would help the breathing.