3 in. hose is indeed nicer, but it creates more problems for an already existing problem- tight confines above the A-arm and ground clearance below the A-arm
I run 2.5 in. hose on my Carrera with IROC narrowbody front bumper. With a stock underbody sway bar, it forces my cooling hose (which is routed THROUGH the A-arm cross-brace via a PVC tube like SmartRacing makes) to hump over the A-arm more and it gets squished against the chassis tub when the suspension travels. I have 23mm front torsion bars, so my suspension travel is less than a stock-sprung 19mm front spring. However with the A-arm through-tubes, it GREATLY helps with ground clearnance. I have no problems with the underside of the duct in terms of ground clearance or debris/off-track excursions causing damage to the hose. Again, where my hose damage occurs is due to the hose atop the A-arm contacting the chassis tub during suspension compression, thus getting a little bit smashed.
I think the key is to use 2.5 in pipe for the A-arm through-tube. 2.5 in. is not a common size and you have to special order it. So for now I am using 2 in. PVC pipe that has some foil tape wrapped on the ends to increase the diameter. You need to increase the diameter because the wire inside the 2.5 in. hose only allows for a little bit of compression when you tighten the hose clamps. Too much compression and it badly crushes the hose and it won't clamp onto the pipe properly.
Nonetheless, I suggest sticking with 2.5 in. hose from the bumper all the way to the cooling plate, with 2 in. or 2.5 in. intermediate A-arm through-tubes On my IROC front bumper, I used a 3 in. to 2 in. plumbing reducer/coupler like this
The 3 in. end is connected to the backside of the bumper via the hose clamp on the coupler. The 2 in. end of the coupler is of course smaller than the 2.5 in. hose. So I sliced through the side of the 2 in. end of the coupler to allow me to insert the 2.5 in. hose INSIDE of the 2 in. coupler end. Works very well, despite what many would say is a substantial bottleneck in the system- the 2 in. PVC through-tube. Bottleneck it may be, but it's WAY better than no cooling at all......
I can shoot some pictures if anybody's interested.