The problem I see is not the speed but the acceleration to get to the speeds needed to catch the B29's. A zeke will acclerate fast to about 250mph, after that u might as well push it to get to its top speed. The only Jap plane that accelerates really quick is the Ki84 and like someone stated, that is only for a very short time. Niki acclerates somewhat quickly to a certain point, then it also will take for ever to get to its max speed. This is the real issue.
When u make a attack on any bomber flying level at full throttle with a Zeke or Ki61, U will basically get 1-2 attack runs on it before u lose all ur speed and it will take to long for u to get back up to speed to catch it. Niki and Ki84 may be the same with maybe 3 attack runs on it and it too will be left in the dust. Now add the Escorts into the mix and guess what, U be lucky to have more then 1 attack run on the bombers for any Jap plane.