For the past 5-10 years or so computers barely got faster, to be honest. They got faster but NOTHING like 1980-2005. Back then there were huge leaps. The past 10 years or so have seen insane size & efficiency improvements. Allowing us to get Mac Pro like performance from 2005 into a small phone at a fraction of the cost, and $5 in total energy costs per year of daily intense usage.
I don't see the next few years to be any different. Everything will get even smaller and more energy efficient.
The Pro series will always be about performance, sure, so it'll get better. But I wouldn't hold out for massive improvements. The 15' is a bit of an exception as the 13' got some big updates, but the 15' didn't. And so the 15' isn't really good value at the moment, still paying a few grand for integrated graphics on the base 15' lol. Ridiculous. So the 15' will likely see some improvements after the summer and make it much better value. But I can't really say how much better.
The reason we haven't seen gigantic improvements in speed is not because it's impossible, but it's much less necessary. The 2005 Xbox, now 10 years old, had a 3 ghz processor. Today a $1400 new Macbook has a 1.1 Ghz processor. Now obviously you can't compare processor speeds 1:1, there's a lot more to it. But that's a genuine choice. We've been stuck with 1 to 3 ghz processors for 10 years now, we added 2 or 4 cores, usually while lowering the speed. The big improvement is efficiency and space. The Xbox was a desktop sized machine with a 130-150 Watt charger. The new Macbook is as slimmer than your pinky finger and its Core M takes just 5 watt and has no fans.
We prefer that over getting even more processing power, 90/100 people don't need extra processing power. Everything they want to do is fine on normal chips. The other 9/100 will use the Pro series. And then there's 1/100 who actually do massive computing, and they use AWS, Azure etc to do heavy lifting. Look at the recent 13' MBA SSD upgrade, it's twice as fast. Nobody really cares, nobody really notices, as once you get into SSD territory everything is pretty much 'fast enough'.
So I'd expect a refresh for the 15' rMBP lineup for sure, not insignificant either. But overall, about your question on the limits of improvements, I doubt we'll see massive improvements in performance, not because we can't but because we prefer improvements in performance/value, performance/size, performance/energy, so that we can bring the Mac Pro performance to things as tiny as a watch, one day. That's where the revolutions will be, making X amount of performance 10x smaller and lighter and more portable etc. Not making X amount of performance 10x better.