.. haven't flown the -800 but it sounds like you are missing the significance of Vmo/Mmo limits.
If you are referring to the barberpole indications, the typical picture is either a constant IAS (or a modest increase) from SL to crossover (at lower levels the aircraft is Vmo-limited) above which the Mmo limit takes over and IAS has to reduce with increasing Hp to maintain a constant Mach ...
... and conversely on the descent .. the aircraft can increase IAS steadily (maintaining constant Mmo) until crossover and thereafter follow the Vmo limit for the rest of the descent
If I have missed your point .. please feel free to ignore the post ...