Naruto is faster in BM but his reaction time is slower
Naruto's reaction time in Sage Mode is faster than anyone else or probably equal to Kabuto(but we are not talking about him right now)
When he moves is KM or BM he is just instantaneous from I guess some miles
What would be a solution to the disadvantages of both of them would be to fuse both Sage Mode and Bijuu Mode. This would fill in all the gaps of:
-He would have the BEST reaction time of anyone because of his Bijuu Mode sensing, the Sage Mode enhancing those senses, and having the ability to sense chakra & dodge it within a nanosecond.
-He would be the FASTEST ninja because it would enhance his Bijuu Mode speed which is already the fastest as of now. The possibilities I would leave up to you.
Now only if Kishi would give us what he had already teased us with back when Naruto confronted Nagato..