The performance? The CPU performance of an M3 Max is essentially that of an M2 Ultra. The M3 cores themselves perform somewhat better, and there are 1.5x as many of them (P-cores). Due to core scaling imperfections, I'd expect the M3 Ultra to be about 1.5x-1.7x the speed of the M2 Ultra or M3 Max depending on workload, rather than twice
GPU performance is not as big an improvement (with the full-power M3 Max having about 3/4 the GPU performance of a 60 core M2 Ultra, and 2/3 that of a 76-core Ultra). That suggests that a full power M3 Ultra will have about 1.33x the GPU performance of a full power M2 Ultra (GPU cores, by and large, do scale).
Where this gets interesting is the "little Ultra" 60-core Mac Studio, especially as a refurbished machine. It'll draw somewhat more power (still highly efficient), but it'll be as fast as an M3 Max from a CPU perspective, and it has a faster GPU. You can pick one up for $3399 (refurbished) in the base configuration, and it ships with 64 GB of RAM.
If you wait for an M3 Max (not Ultra) Mac Studio, I'd expect "big Max" (the 16/40 model with competitive CPU performance) to be almost that expensive. The MacBook Pro took a $200 price jump in the base configuration from M2 Max to M3 Max, so I wouldn't be surprised to have a base Mac Studio around $2299 or even $2399 if Apple tacks an extra hundred on to the lower-production Studio. Because the difference between the two M3 Max versions is so significant, Apple charges $500 extra for the fully enabled one (and a RAM boost from 36 GB to 48 GB). That suggests that a "Big Max" Studio will start around $2799-$2899. Give it 64 GB of RAM and it's $2999-$3099.
For a $300-$400 price difference in the same configuration, is it worth buying a discounted M2 Ultra now instead of an M3 Max later? You get a boost in GPU performance, extra Thunderbolt ports, a theoretical boost in RAM access (for the VERY rare scenario where 400GB/S is not enough), and a computer now, instead of in 5-9 months (everybody hopes for WWDC, but October is a real possibility).