I haven't stepped down to OfficeXP on Win8, but I did load Office2003 on it successfully. I have a feeling XP will probably work OK, but ?? there's no good test except "doing it", alas.
We are keeping XP boxes up and running because some of the software is GREAT and "newer versions" castrate out the useful services. Those wouldn't be upgrades AT ALL, therefore - just newer, shinier things that don't do as much but cost more! "Why bother?!! What's the REAL purpose - just spending money?!!"
When I've taken a 2Gb RAM XP machines into 4Gb, I can 'feel' the difference for a few days. After that, no. It might be 5% 'faster' at times, or 10%, but only for Loading Software moments, or Save moments. But few if any other moments. How productive am I being when I am constantly "loading new software"? Uh. Not very. I CAN be productive doing SAVEs. But how often? Ten times an hour? Fifty times? Well, I wouldn't be doing MUCH work if I was saving so often.
So "Save Time Benefits" are rather minimized, as well.
When you mentioned MathCAD, then I'm thinking, "So, does the newest version of it make 2 plus 2 equal - uh - something better?" No. It probably has been re-written to benefit from multi-cores better, but how much? If you're making $200 an hour of those results, perhaps saving 5 minutes out of 60 is 'significant' but I'd consider those additional revenues compared to the new MathCAD costs.
Plus, if my computer was a moneymaker or used for productivity, I might hold off on a move to Win8 for months and see IF you start hearing "Win9 news". Win9 will probably want a new MathCAD as well - or, at least "will benefit from a new version".
There were public discussions of "Win7" within a few months of Vista's delivery, and obviously Microsoft has teams working on Win10 and Win 11 now (probably more distant ones, too), and while Microsoft might complain about these "leaks" as injuring their immediate revenues, Microsoft Customers might thank them by saying, "At least you didn't make us spend the same dollar twice - now, we can spend two good dollars instead of one bad one."
For Developers, they don't just spend a few dollars. They spend hundreds and thousands of hours, and the folks that had to develop Vista-drivers were very unhappy with Vista's performance problems. They may have held the whips to speed up Win7's delivery, in fact - we see "Vista-Win7 drivers" all lumped together for SOME reason. And that's the developers' behind that.
As for laptops... I haven't seen a lot of changes in prices, and that's odd. An i5 notebook was going for $450-600 in their first year of delivery and, now, 3-4 years later, I can open the ads and see "i5 - $450-600" still, and their speeds are all basically the same, too.
The notebook market seems to have set itself in concrete, almost. The low-end is $250-450, too, with i3s taking the high-end and everything else (1.3gz dual cores!) taking the low-ends.
That's really the battleground of typists. "Do you want to type and do productive work, or do you want to sit back and be spoon-fed entertainment?"