The only downside to the GP4000s is perhaps a very slightly less supple ride than other tyres. But this is more than made up for with puncture resistance, wear, grip and low weight. If using alloy wheels then coupled with latex tubes would improve the ride feel. With a C17 rim then a 23mm is the optimum for racing. A 25mm ends up with a lightbulb effect so although it may be more comfortable isn't faster. I don't think you can call it a racing rim if the inner is wider than that.
There are plenty of racing rims with substantially wider internal widths, the newer Roval wheels, ENVEs, Bontrager Aeolus. I don't think I regularly ride any wheels under 19.5mm internal width any more.
The fashion in recent years is to go wider and that is the perception for a lot of the consumers that wider is faster. Only if coupled with the right tyre. A C15 with a 23mm tyre is no different than a C17 and a 25mm if comparing the profile of the tyre and rim. Too wide a tyre on a rim causes it to lightbulb so losing the aerodynamic effect. Which is why a 23mm Conti or Vittoria (measures at 25mm+) fits so well with a C17. A Conti/Vittoria 25mm (which measure over 27mm) starts to lightbulb. But then you can start to mistake larger tyres having a greater volume of air in them, thereby being able to run them at lower pressure and that being more comfortable and feeling faster. So for training comfort is possibly the priority over speed so a bigger tyre is always better but if it's outright speed you're after, and if you're racing that's what you want, then comfort is sacrificed a little for the aerodynamic benefits. Otherwise you would see the pros running 40mm tyres and mountain bike rims at Paris-Roubaix. They only really go up to 26/28mm tubs but then you can't compare clinchers and tubs at all when it comes to rim width and the effect on tyres. Rim width has no effect on how wide a tub is but has a huge effect on the profile of a clincher. Bigger internal width of rims need bigger tyres to maximise those aero benefits.
This article that's been used here before explains it far better than I ever can https://intheknowcycling.com/2016/04/03/best-wider-road-bike-tires-wheel-sizes/
I think the differences we're getting with a certain rim width, tyre width, pressure and so on are far less significant than the manufacturers would have us believe. Anything to get us to buy the latest as if what we're currently using is now no good.