Reason for this is that most lammy' have a weak spot just over 1/4 throttle when cruising at high speed or coming down from full throttle at high revs.
This is a lot harder to jet out with a Mikuni or Keihin as they only have interchangeable needles and a fixed atomiser where as with a Delly you can change both.
Also remember that the bigger carb size you go the less signal you get from the engine which also effects throttle response.
As it says in a 2 stroke tuning book i've got "its like sucking through a straw. the bigger the diameter of the straw the harder it is to suck the drink up. The same is happening to your engine."
No sucking jokes please!

My opinion only......i dont want to start a war on Mikunis v's Dellortos!