Lately I've been driving the BMW M1 Procar and the Volvo 240 Turbo and it seems like it's impossible to rev-match on downshifts. Even if you blip with 100% throttle, the wheels still lock up and makes the car spin. It's especially apparent on the Volvo when going into 1st gear.
Try to slown down a bit more before engaging 1st gear. Even if you blip the throttle before downshifting, if the speed is too high you will stil lock the rear wheels
With a lot of cars, first gear is only for getting you out of the pit garage and off the starting line. So it's a very high ratio. Once you're up and running you should forget it's even there.
I agree for 1st gear, but for 2nd, 3rd and so on on the Volvo, even blipping with 100% is not enough. In fact, it almost feels like I'm not blipping at all.
I just drove a few laps to see what was going off and couldn't reproduce any problems. See video below: What pedals do you use? I can recommend adjusting the sensitivity and deadzone of the clutch to aid downchanges.
I use Clubsport V3 pedals. Seems like you're downshifting in a straight line, where that issue doesn't appear. Try doing that while cornering, it's impossible not to spin. I don't think it's down to my driving nor my pedal settings. The car was fine in that regard before the update. Furthermore, I have no problem with the Nissan or other cars with big turbos (Capri, BMW 320 turbo etc)
I am also using Clubsport V3 pedals. Downshifting in a straight line is indeed the correct technique, especially with something which has fairly widely spaced gear ratios. It is correct that that downshifting while turning in will cause you a plethora of issues, and in the real world it's something that I've always taught people to avoid. Gear changes should be done in the mid third of the braking zone, so that they're all done and dusted before you start to load up the tyres laterally. Of course you have examples where you simply can't avoid it, such as corners which progressively tighten, however they tend to be quite rare. The correct technique there is to pick a place in the corner where you can momentarily unload the tyres laterally to get the gearchange done.
During the recent Formula Raceroom Junior @ Brand Hatch Indy competition, I was able to downshift to 1st gear in the first part of the Druids bend, while still braking with heel and toe. It allowed me to have a good corner exit speed but it was really acrobatic, with only one valid attempt out of ten tries!! The same for the downshift to 2nd gear for Graham Hill bend, where the rear end wanted to take the lead... The Volvo and 934 are also very sensible to downshift and as Alex says you have to choose the right time and really push the throttle to have some revs and avoid lockups.
I still don't understand why it's so much worse than the Skyline or any of the Group 4/5 cars. Sure, downshifting mid-corner may unsettle the car a bit but not that much. Judging by the engine note, it's almost like downshifting without heel and toe.
It may be because the gap between gears is more important; the 934 has a 4-speed gearbox whereas the others have a 5-speed gearbox.
Where possible I always reduce the spacing between the gears It helps me a lot, group 5 for example, I do not have the car you mention so not aware if that is possible