The plus should never overshadow the regular size. The "regular" size has been the flagship forever, so once you start forcing people into spending $100 more for the true flagship, they will get mad. Especially since they will need to deal with a screen size that could potentially become inconveniently large.
That being said, I don't think they should have identical specs. The plus size should be *just* enough more powerful to perform the same.
Once they squeeze 2GB of RAM into the iPhones, there won't be as many issues. 2GB of RAM in both devices alone would fix a ton of things. (I've heard iOS 8 caches the animations in the RAM, so that would fix a ton of lag issues) If they clocked the plus just a bit faster to deal with the pixels, on top of the extra RAM present in both phones, things would work perfectly. I'm pretty sure this sort of thing will happen with the 6S series.
Yes, I'm sure the 6S+ will be everything the 6+ should have been and it's a reminder to everyone that the S cycle is the one to be on. I haven't had an 'S' phone since the 3GS but have surely learnt my lesson with the 6+. Apologists can say all they like but for me the 6+ was incorrectly specced out of the box and six months later they still haven't managed to fix the issues with software updates. Sure it's a good phone in certain respects but has such glaring flaws which render it a PITA.