Many developers aren't interested to Apple's "new fluffy colorful things" but the web browser improvements. Here is an unofficial list about what has changed in Safari on iOS8 (*).
So let's do some benchmarks, to test WebGL performance and the rest.
Web standard support: html5test
The most known benchmark to test the current web possibilities.
WebGL 3D performance: aquarium
Fps: 20 (with 10 fishes)
A simple WebGL demo by Google, the newer iPad should have much better results
Canvas 2D performance: fishbowl
Fps: 9 (with 250 fishes and water/sound disabled)
This is a demo from Microsoft
score: 1316 (it's increased of ~100points after the upgrade from iOS 7.1 )
The v8 benchmark is old and it works everywhere, the new Octane 2 and 1 crashed on iOS8 on iPad2.
In conclusion I can say that the web stuff has improved in iOS 8 but if you have other tablets please share your results in the comment!
(*) Personally I don't see the "upload bug" because I can upload files via web browser on iOS 8.