928 is exactly the same as 925 and 920, except for AMOLED vs LCD (920) and Xenon flash. It should be just as snappy, it has the same ship, same RAM same memory even.
I know how WP8 works on 620 and 720, and it's just as smooth (with a little extra loading times) as my 925, but the 1520 (same HW ad 930) I tried once (brand new) was super smooth, practically no loading screens. Yes, WP flies on quad core, but it still flies on dual cores as well.
1520 and 930 (and Icon) are the new flagships of WP, but 92x and 1020 are still going strong. Remember that most selling WP devices are the most low specd, so if developers are smart, they'll optimize their apps for those phones, which means you can easily enjoy your 928 for an extra year (or more) and get the 935 or whatever is released next.
If I had money to burn, I'd preorder the orange 930 right away (but only if it has glance), but I don't, so I'll stick with my 925 for another year and a half until my contract runs out. I'll see what's new then, and I suggest you do the same
And yes, you'll get the latest and greatest WP and all the updates to come on your 928, and once you get the extra row of tiles and that wallpaper, it'll feel like new phone again
And you have xenon flash on 928, which is 1000x more superior to LED flashes. Enjoy it until it lasts.