Well it's a weird question anyway. XMP isn't overclocking, if it works, it literally IS stock. It's meant to get the RAM to the manufacturers tolerances. A 3200mhz stick will run at 3200mhz, hopefully with the timings you spent more money to enjoy.
So what you do mean by stock? My 4X16GB sticks on my zen1 CPU would never run XMP, and I could only get 2666mhz out of them with a manual tweak. When I stuck a zen3 chip in the same 6yr old board, I can now just enable the XMP profile and it's rocksteady.
If you mean just sticking the ram in and leaving it, that's NOT stock because every CPU generation handles RAM slightly differently and therefore might default to a higher or lower "minimum" memory clock. My zen1 wanted my ram at 2133mhz, but zen3 without xmp would set it automatically to 2933mhz... because mobo manufacturers also make their bios's handle RAM differently...
So, XMP is the definition of "stock", anything less is underclocked, anything above is overclocked. You can also get the exact same settings manually as XMP.
So to answer your question, you would need to tell us what CPU you have (maybe even which stepping), which mobo (even which version) from which manufacturer, and which RAM with which specified achievable settings... and there's no official policy, so you'd have to buy the whole system and tell us what initial speeds you are getting before XMP. ALSO every piece of software reacts to memory in different ways. Some like pure mhz, some like tighter timings, a lot don't care what you do... So it would also be really helpful in that case if you could run a load of benchmarks on your new PC with XMP on and off, and give us the numbers. Only then would we really be able to give you a percentage difference...
Obviously I'm being facetious... the short answer is your question doesn't really make sense, and if it did, there's no way to know without hard evidence.
Edit: but definitely enable XMP if you can, and don't want to manually overclock. You at least will get the performance you paid for, and not... less.