TDP numbers have started to mean something else in the past couple years, but in that era, it would've been fairly spot on. At this point, TDP is more about a target and the actual power output and cooling needs are largely up to the needed performance and the design of a particular system or implementation, at least on mobile. They're closer on desktop, mostly because most wintel-focused desktop implementations end up designed and cooled similarly. I think most of what "TDP doesn't mean anything any more" really has to do with is letting chips turbo further for longer than before, creating scenarios where a chip whose TDP is, say, 15 or 28w might run at 35w full-time if it's cooled well and you load it down with work. That's not bad necessarily but you can no longer take TDP as the maximum a chip can output like you could basically up until 2011-2013.
One thing to be aware of is that GPUs got good at idling and reducing their own power usage years after CPUs did, so a MacPro 4,1 or 5,1 with its stock graphics card is likely a fair bit worse at conserving energy and not putting out unnecessary heat than, say, one with an upgraded GPU. So if a 2008 Mac Pro had a basic graphics card like a GT120 and a 2010 one had a high end one like a Radeon 5870 or similar, that might explain some of the difference in power usage. There's also the rest of the peripheral loadout and configuration, and things like how well or if cooling/thermal interface materials was maintained. i.e. a machine that's pouring heat out might be doing better for itself than one that isn't pushing the heat out, but, that's all highly dependent on lots of specific context information.
The Mac Pro 1,1 with an original/early high end video card is a "great" heater, especially if you have a fairly early CPU stepping, as even within a particular CPU model's run, different steppings can have different thermal and power properties. I'm told that better-on-power Xeon 5100 series chips do exist. The gotcha of course is that the MP1,1's high end video card options aren't that reliable. I've got two failed Radeon X1900XTs I intend to replace at some point with something stock from a 4,1 or a 5,1.
To be honest, the QuickSilver probably wasn't doing much of the warming if you alsoleft three or four big LCD displays on at full brightness. As noted, G3s and G4s use almost no power, and if your Mac has any power savings enabled at all, it wasn't spinning its hard disks, and G4-era graphics cards don't draw all that much either.
My QS'02@800 barely puts out heat at all. On its own, it wouldn't be able to budge the temperature of a bedroom.
