The upgraded processor is essentially identical (except for clock rate) to the slower one, except that it has a bigger L3 cache. How these changes translate to performance is not a simple question. With the best answer being “it depends”. In reality you would be unlikely to ever see a full 10% performance gain in all but the most contrived circumstances. The bandwidth to memory in unchanged, and for most compute intensive tasks this remains the most important bottleneck.
The reason these processors exist is three fold. One, Apple, like everyone else, need a product at the most premium price point, simply to sell to those that have the money, and feel the need to spend it. Two, there will genuinely be people that want the most bleeding edge performance. Where they need it is another matter. But the chips exist, Apple would be remiss not to offer them as an option. Three. The chips exist because the chips that come off the production line are tested for their operating range, some will work fine at higher clock rates. These are marketed at a premium price. Also, chips will often be defective, due to various glitches in the manufacturing process. In the past that simply meant it was useless. However nowadays L3 cache accounts for the lion’s share of die area, and the most likely place a defect arrives in in the L3 cache. So if you design the cache so that defective areas can be disabled, and the remaining good cache used, you can significantly increase your yield. Thus you may find that a 6MB chip is actually an 8MB chip with a defective cache bank disabled.
OK, but should you spend the money? Probably not. If you want to do movie work, more memory is most likely the best was to spend money. Apple have actually got more sensible with memory prices with this new release too. Still a premium, but not insane rip-off they were. The other, spendy, but spectacular way to get performance gains is a SSD. For movie work this could produce astounding gains. You might want to check in with some of the professional forums for opinions.
If you are doing movie or other visual work, I would be inclined to spec the anti-reflective screen option. Yes it looks a bit less snappy, but most visual professionals find the glossy screen a significant problem when critical work is done. That is the reason Apple still make the anti-reflective screen available, and after much protest, have reintroduced it on some models where it had gone.
The newer 17" machines are huge. I have one of the original PPC 17" machines, and it is only slightly bigger than the new 15".