The top of the line M2 Ultra 76 core GPU has 1/4 the performance of a 4090.
Not going to argue that the RTX 4090 doesn't have the crown when it comes to pure rendering, but my M3 Max MacBook renders Blender scenes faster than M2 Ultra—so the M2 Ultra isn't the benchmark.
The M3 generation got hardware raytracing, so it will be interesting to see how Apple might configure an 'Ultra' or 'Extreme' chip, now that we can't assume that M3 Ultra is two 'Max' chips fused.
I'm not holding my breath, but I'm also not ruling out that Apple has a Mac Pro configuration in testing sprung from a "hold my beer" moment of inspiration, with a corresponding price tag.
As nice as that would be to see as a halo product, the market would be extremely small. I think Apple shines the most in the enthusiast market segment which has some money to spend for nice solutions; pricey to many, but not 'strictly business 2 business' prices.
All that said, it's a huge brain fart to think that just because a PC with a 4090 (or two, or four...) renders a frame several times faster than a Mac, it's those several times better. The 'wow' factor is there, for sure, but that's not how I have ever experienced computer work, even 3D work.
During my 5.1 days, for a long time I had a Radeon R9 280X. At the end for a brief period I had a Nvidia 1080 Ti which I could use for GPU acceleration during rendering. Massive speed improvement. A few times I just started a render in Maxwell and watched it go. Like staring into an aquarium. That's not working, that's waiting. It was satisfying at the time, but I was done with 'watching renders' pretty quickly.
At the end of the day, what counts is the overall experience with a focus on "viewport performance". Sure, today we do get realtime raytraced feedback during lookdev. But I get that on my Mac Pro and MacBook Pro too. There is nothing about today's Macs that keeps me from doing 3D work. Nothing. If I have a personal project I'd like to take to 'final pixel' I'll queue those up for when I'm done for the day. Commercial work: render farms all day.
Not saying I wouldn't take a Mac that was 6x faster, just pointing out that perspective is really important.