Higher end components will let you shift faster and at higher loads. However, it's not exactly like you're going to lose a tremendous amount of momentum in the quarter second it takes to ease up and shift. If you're riding in a competitive group and want to respond to an attack up a hill, then Sora is going to force you to remain seated and soft-pedal until you shift down a gear or two before you stand up to respond. With Ultegra I can shift while I'm coming out of the saddle. It will still kick and let me know it's unhappy but it will shift unlike Sora. I've gotten stupid a few times doing this and almost sent myself over the handlebars because I didn't until the gear was engaged before really pushing it. It still shifts, but that much jerking is not good for you or the bike. I've ridden SRAM Red and it is noticeably better than Ultegra. Very quick shifts and it seems to shift under moderate load much better although I didn't go crazy on it since it wasn't my bike to abuse. The holy grail is of course Dura-Ace Di2. Never ridden it, but I've watched people shift it while sprinting on a trainer.

That's an amazing engineering feat; no other groupset comes close. There's a bigger difference between Di2 and mechanical Dura-Ace than there is between Sora (lowest end) and mechanical Dura-Ace. In the real world, however, my Ultegra is more than sufficient. You really don't need to be able to shift while hammering.