Originally Posted by
reppans
I think the issue is the gear overlap and Brompton's odd IGH/derailleur system.
Indeed. You could achieve at least an interesting setup with tiny steps and short gears for some by using 12t and 17t sprockets on the back on the BWR and 36/44 on the front:
Bicycle Gear Calculator
But it would be complicated to remember which of the then three shifters to shift when.
Originally Posted by
reppans
The double chain ring mod looks good for riding/touring in mountainous areas viewed as low range set of 6spds/~25% steps for long uphill slogs, and a high range of the same for the flats and downhills. But for the ultimate Brompton gearing, I think the OP should do the 3cog rear derailleur mod for a low/high range 9 speed with even-ish ~18% steps...
Jupp, that's my opinion, too. To add a second ring as a granny gear for steep hills is always an option. But three shifters for general riding are a bit of a pita if you ask me.