Toyota Repair: 4Runner performance, toyota 4runner sr5, performance exhaust system

Question
Ted-
I drive a 1990 Toyota 4Runner SR5 V6 with about 133,500 miles on it.  It has an automatic transmission.  Although it has adequate power most of the time, when I go over mountain passes or try to accelerate on certain inclines, I have trouble getting it to accelerate.  On these inclines, the rpms dont want to go above 2.5-3 thousand.  Only when I push very hard on the gas pedal does the engine speed up, but when it does this it suddenly jumps to 4-5.5 thousand rpms.  This doesn't seem like it's very good for the engine and transmission.  On inclines at highway speeds, it sometimes reaches these high rpms for only a second, then downshifts again, making it hard to go over 50 mph on steep inclines.  In normal driving conditions, the engine and transmission seem fine, I have no trouble accelerating quickly and don't sense any horsepower loss.  I have compared it to an identical one with low compression and mine performed noticably better, making me think compression isn't the problem.  I was wondering if this sounds like a problem or if it's just because of the lack of power in the older 4Runners or the gearing in the automatic transmission.  I was wondering if there are things I can do to make it shift more steadily and easily on inclines.  I think my spark plugs are getting old and my spark plug wires have never been changed, and I was wondering if that might be contributing to this.  Also, I am planning on getting headers and a performance exhaust system with a high-flow catalytic converter, and I was wondering how much of an imporvement those will make.  I would really appreciate any diagnostic information and/or advice you can give me.  

Answer
Sounds pretty much normal, I've been driving a 1990 4 runner for about four years now and know exactly what you are talking about, these engines are not very powerful, changing the spark plugs etc. probaly won't help.
I have no idea if there would be an appreciable amount of performance improvement when changing those parts.