You won't notice that much difference, I wouldn't do it. Waste of $ IMO. And as always, wider doesn't automatically mean slower. I think you should elaborate a bit more on what you mean by "Allround" and "Dry/Fast". Riding for fun vs racing? Wet vs dry? Etc.
As far as I know and in my experience, when riding offroad more bump absorption and a smoother ride usually means faster too. And you also see those road bike tyre rolling resistance tests where they compare tyres from 23 to 28-32, and the biggest tyre comes up on top, but i'm not smart enough to explain that in detail.
If you don't even have the frame yet, don't start planning this kind of stuff. Wait until everything arrives, build the bike, and ride it for a while with those 38mm G-Ones. I do this all the time, but the researching on the internet vs riding the bike in real life are two very different things.
Ride the thing, gain some experience. Without experience you'll just go mad and get lost in all the options and the opinions of different people. You have the G-Ones, and 20mm ID rims work perfectly with 35-40mm tyres. Just ride that combo for a while and then come back. Don't order a second wheelset and tyres blindly.
Supported videos include:
Please paste your code into the box below: