It's not much, that's for sure. In Anandtech's SPEC2017 testing, the differences are as follows (they tested ADL with both DDR4 and DDR5):
Normalizing for clock speed (they are all likely to maintain peak boost in these ST workloads, so 5.2GHz vs. 4.9GHz):
12900K D5: 1,56538 pts/GHz INT, 2,72308 pts/GHz FP
12900K D4: 1,54038 pts/GHz INT, 2,62885 pts/GHz FP
5950X: 1,56122 pts/GHz INT, 2,48776 pts/GHz FP
Which, using the 5950X as a baseline:
ADL D5 IPC: +0,3% INT, +9,5% FP
ADL D4 IPC: -1,3% INT, +5,7% FP
Of course this is just in one set of workloads, but at least SPEC is an industry standard. The numbers will obviously be different in different workloads. But it's very close overall.
Yeah, this is where the difference between what TDP actually means vs. what it is understood to mean comes in. After all, TDP is
actually a spec for OEMs and the like to say "this is the class of cooler you need for this chip to maintain stock performance", and it is divided into classes for simplicity rather than calculating an accurate TDP for each chip (as that would be a complete mess in terms of coolers). That's why you get those "54W" Celerons running at 30W, and so on. Something similar is true for AMD, though the problem here is that (just like Intel) they've also used TDP numbers as public-facing marketing classes without really explaining what the numbers mean, or even working to make people understand that TDP does
not mean peak power consumption. That the relation between TDP and power draw (to the extent that there is one at all) differs between the two manufacturers just makes this into even more of a mess.
I was initially glad that Intel had ditched TDP for their two-tier base/boost power system, though it seems that is effectively worthless for K SKUs, with boost power being the only number to look at. It'll be interesting to see how this plays out across non-K SKUs though, and I would love to see AMD be more transparent as well.
Heh, I kept my Q9450 for nearly a decade (2008-2017!), and it served me well the entire time. If you buy a good CPU to begin with, it can last for ages. I only upgraded my 1600X to a 5800X because I could get it funded through work, otherwise that chip (which now lives a new and better, calmer life running my NAS) would have stayed in my main PC for at least a few generations more.
Still, as I said, I would keep my options open and take a close look at the upcoming Zen3 V-cache chips. I generally dislike Intel (mostly due to their long history of shitty business practices), but obviously make up your own mind from the factors that matter the most to you - both major CPU manufacturer deliver excellent performance and great platforms these days.