However, I can't see why a modern equivalent of the 20-30 would require a larger supply if they are meant to be more efficient.
Because the majority of existing gas pipework I see is undersized. The majority of boiler installations did not (and often still don't) comply with the regulations.
So even your existing boiler may be installed on undersized pipework; if the boiler gas valve has a test point this can be verified.
When I installed boilers if the pipework looked marginal I would install the new boiler and then check the pressure drop....if too great then I would then upgrade the pipe. You just make sure the customer understands this from the outset. Cowboy installers keep this quiet since it allows them to undercut the competition despite being illegal. The boiler may then appear to funtion correctly to the homeowner despite possible safety implications.
The gas pressure from the meter is approximately 100 times LOWER than a car tyre pressure, therefore friction within the pipe and fittings will cause a significant pressure drop if the pipework is not adequately sized. 35mm pipe is approved for domestic work and is sometimes necessary.