An SU carb `resizes itself' by lifting the piston in responce to the airflow required.
Even an HS6 can give good low speed tractability, as the piston doesn't lift far under these conditions.
What you propose has some merit with fixed venturi carbs, but with SUs I believe it will just add complication, not power.
[edit] Progressive twin choke carbs eg Weber are a poor attempt to achieve what an SU does with its 1 variable choke.
BTW I do like Webers, but not the progressive ones (been there in a pre-Mini life with an Italian Love Affair).
Weber 45DCOEs, (and Dellorto 45 DHLAs)- now you're talking...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
