An interesting discussion..apologies,havent read all of it but will just add that IMHO...
Adding a larger pipe diameter anywhere along the run will lead to a decrease in pressure loss due to the reduced resistance to flow through that section of pipework. Of course,from a design point of view, we should be starting with the bigger pipe diamater at the meter and reducing as/when necessary.
What you do need to watch out for on long runs is the discharge volume.
For example...and to illustrate, if we have a known pressure at the meter outlet,say 20mb,connected to a length of 15mm tubing..lets say 10m..then there is a finite discharge volume of gas which will be available at the end of that pipework. You can find tables in various documents for discharge rates of given diameters of pipe for a given pressure differential.
So,in our example..if we have a great big length of 15mm and we then stuck in a section of 28mm on the end of that in the hope of reducing the pressure loss..we may still be on a loser because the discharge volume on the end of the 15mm length could well be insufficient to feed our appliances !
Meter outlet WP is typically 20mb +-2mb. It can be lower than this and often is due to network management and other issues.
It is set this way as most appliances save for central heating boilers, have no internal pressure regulation and so for example,cookers and fires rely on the meter regulator to maintain a safe working condition.
If your tempted to break the seal on a meter reg and wind it down to try and get your new combi to work better, think about over gassing the other appliances !
Once upon a time everyone had gas fires and cookers,then they had "normal" natural draught central heating boilers.
Now everyone wants a Megawatt combi.
No one is throwing lots of new pipework in the ground and increasing the diameters/pressures of the supply network so something has to give.