Few percent if anything, nothing overwhelming. If needed you'd already know, save your $$$. Smart money is on waiting for the reputable reviews before you make a decision.
Also worth noting a delay also allows Apple's providers to settle the manufacturing process, of which there are always issues initially.
Q-6
I'm not sure this is great advice. We saw definite improvements in the mid-2019 models moving from the i7-> 2.3 i9, but less so from the 2.3 i9 -> 2.4 i9. Power draw on those machines was about 45-50 watts in sustained workloads. If Apple is right and this thing can draw ~20% more power, we could see the chips differentiate themselves more, particularly the i9s from the i7s. Many of the "good" benchmarks (likely run after the system stopped indexing itself or doing other background tasks), showed all versions obtaining and maintaining some kind of boost frequency for a cinebench run (often around 3GHz for the i7s and i9s). Which makes a ton of sense, given that cinebench scores were a bit less different than the math (2.4*8 vs 2.6*6) would imply. So the 6 core was boosting a bit higher than the i9s, but not enough to overcome the 2 core disadvantage. The difference between the 2.3 and 2.4 i9s though was very slight.
I suggest people go check out this thread if interested in the same chips in the old body.
As for how it impacts this decision. If you are single core workloads, the top turbo boost of the i9s is a something to be very aware of. A single core work load I'm guessing won't be thermally constrained, so you should be pegging the top turbo of the chip most of the time. That's 4.5, 4.8 or 5.0. So $400 gets you ~11% faster CPU for the life of the machine in single core workflows. For multicore, the benchmarks will tell us if the added 12Ws is both real and how much it impacts the i7 vs i9 choice. Previously, the i7 was a touch closer to the i9 than it should be on paper, but that might change. It should be about a 23% difference (2.4*8 / 2.6*6), but higher available wattage might impact things in either direction, it really could just come down to the efficiency of each chip at maintaining what turbo at that power envelope.
The other practical consideration is that the 2.3 i9 is not available without the 5500M and 1TB of RAM. So value seeking CPU users might be inclined to pick up the 2.4 i9 CPU upgrade on the base model and leave many of the other options unchecked. I'll likely do this for example, because the GPU is largely wasted on me and spec-ing out the base model to match a slightly upgraded top model suggests that 5300M -> 5500M upgrade is costing $100.
tl;dr: The CPU upgrades are almost certainly going to have some benefit, but it might be slightly less than what the frequency math implies. Decide for yourself if its worth it.