John Cutler
If you increase the back-to-back, then you might need to increase the check rail gauge.
Otherwise you increase the risk of wheels catching/derailing/riding up at the crossing nose/frog.
My advice is to stick with the EMGS standard of 16.5mm unless you want to do some testing or further research.
Many years ago Bernard Weller sold an EM back-to-back gauge of 16.7mm (which I use!) so it is possible to increase the BB and apparently so do other EM users. But I use a check-rail gap of 0.9mm (aluminium strip) instead of the 1mm per the EM standard (even this is anathema to the purists). Supposedly the increased BB gives less ability for rolling stock to yaw (twist) on the track but I am not sure it is worth the extra effort (but it is not a lot of extra effort unless valve-gear clearances are involved!).
GW Models (George Watt) advertises an adjustable BB for £15 in MRJ266 (tel 01903 767231) if you really want to go down this route.
I certainly agree with Nick about the inability to measure track variations to within 0.15mm (well I hope I am within 0.1mm!!).
If you take 2 supposedly identical gauges they will often differ.
You need to spend an awful lot of money to produce or buy an accurate gauge.
There is a current debate about accuracy of gauges and the pitfalls of dependency on them in the ScaleFour Society.
Health Warning: Some people get quite vitriolic when people deviate from the accepted standards!