Ju188A-1 could do better than 330mph, a substantial improvement over the Ju88. The problem wasn't that it wasn't a big improvement, it was that the Germans were so pressed for production they couldn't spare the time to retool their production lines. That wasn't just a German thing either, big improvements to things like the Lancaster, P-38 and F6F were passed over due to the production delays that would have been incurred.
The Ju188 was superior to the He177 because when you sent 20 Ju188s on a mission with no enemy opposition you could reasonably expect that more than 12 of them would actually complete it whereas not so much with the He177. Yes, a higher percentage of those He177s that didn't suffer mechanical failures would survive, but if you are looking are overall failure to complete the mission the winner is clear.