Probably not reviewed because it's more of a hack and not a common use case for the Apple Watch.
But to answer your question, the display on the Ultra actually isn't much bigger than the display on the 45mm S7 or S8, so I can't imagine it would be noticeably better. Keep in mind that the '49mm' or '45mm' is referring the size of the case, not the display...and the Ultra has thicker bezels.
Here's the breakdown on the different display sizes:
49mm Ultra is 1164 sq mm / 410x502 pixels
45mm Watch is 1143 sq mm / 396x484 pixels
41mm Watch is 904 sq mm / 352x430 pixels
So the Ultra’s display is only 1.8% larger than the 45mm watch (or 21 sq mm / 14x18 pixels). For comparison, the 45mm watch is 26.4% larger than the 41mm watch (or 239 sq mm / 44x54 pixels).
Also, I don't think the flat screen would have anything to do with it being quicker or more responsive. It's probably just your SE being slow. My 41mm S7 is quick and responsive, and obviously doesn't have a flat screen.
Yes, but as mentioned, it's more of a hack. Send yourself a link in messages and tap it to open the site. There's no browser app that you can go to directly.