Easily enough. 4.8GHz is a very modest overclock tbh, especially as the CPU's single-core boost frequency is already 4.7GHz.
The Noctua air cooler is more than sufficient. The H80 is in a similar ballpark to the Noctua. The H100x is better than either and more than fine for an 8700k with an overclock.
Moreover, PCS test the systems before shipping them out with extra testing for OCed systems to make sure they are behaving.
What I would say is that Intel are releasing new CPUs in October that will be clocked higher out of the box possibly even than an OCed 8700k, so if it's not an urgent purchase you may want to hold off for those.
5GHz or above would be more of a stretch. A few more tidbits with the OC:
- some motherboards will actually set the all-core multiplier to 4.7GHz anyway (normally 4.3GHz 6-core boost) without the user asking it to - because it's that easy to squeeze more performance out. PCS's OC is only just above that.
- it's upto 4.8GHz - so if 4.8GHz is not achievable for your CPU then they'd just go as high as was stable and safe rather than forcing 4.8GHz
- They only charge £10 or so for the pre-overclock. So while it's a decent and almost-free performance boost, potentially giving as much as 500MHz more across 6 cores, they're not going to be selling a truly extreme overclock for a tenner! If it wasn't pretty straightforward I expect they'd charge more.