5'10" with a +4 ape index myself. Feel it definitely helps me, but also hurts me. I'm pretty skinny, so moves that are very tight are very hard for me. I'm not built for that stye of climbing. Plus my lock-off strength isn't particularly great. My main climbing partner is around 5'6" with a +2. You'd think I could outreach him all the time. There are times where that is the case, but he is so damn strong and can lock-off well past 90 that he can often reach further than I can. Where I have him are on long dynamic moves.
As far as hitting a plateau. Agree that it's only natural. The progression curve in climbing (as well as many other sports) is definitely sharp in the beginning with lots of gains and very slight as you progress. The gains I've made recently have occurred over the past years, whereas I could see gains in a matter of a few sessions when I started. I also feel like I am getting to an age where I'm probably close to my max outside of doing some serious focused training and/or getting my hands on something like HGH. But I echo [MENTION=143526]theesotericone[/MENTION] above. That would suck the fun out of it. I do quite a bit of focused training now, but it is all geared towards either injury recovery or prevention. The joys of getting older!