It's a bit quiet..

[MHN-RS]

Is there a potential issue with messing with the filler neck on our more recent RSes messing with the EVAP system? I know overfilling on a Multistrada causes raw fuel to dump into the charcoal canister and cause problems.

The depth of the filler neck corresponds to what BMW believes is the highest fuel level that protects against fuel overflow into the two-way tank vent path, which includes the charcoal vapor capture canister.  Some may get away with modifying the filler neck, particularly if they also remove the charcoal canister, but the canister is not the only concern.  

The consequences of fuel flow into the canister are plugging of the tank's vent, which can lead to the engine stalling because no air can enter the tank to replace the fuel the fuel pump is drawing out (at some point the pump can no longer overcome the vacuum so no fuel goes to the engine).  K12LT's were notorious for this, as the initial ones had their canister atmospheric vent hoses located directly over the rear tire.  Rain water thrown up by the rear wheel would get sucked through the hose to the canister, blocking the vent path.  For those that do not believe much vacuum would be generated in the tank:  one of the signs of a blocked vent was a fuel gauge that never went below 1/4 -- the plastic K12LT tank would actually deform inward so much that it would deform the fuel level float tube at ~1/4 height.  The same symptoms/problems appeared when overfilling the tank with a too-much-modified filler neck.

The other significant concern is overflow onto the ground resulting from thermal expansion of the fuel.  The stock filler neck height provides enough residual air volume to accommodate such expansion without pushing liquid fuel out the vent hose.  This is not a theoretical concern.  One  example: the latest generation Gold Wing has a smaller tank than the last model, so some have experimented with modifying the fuel neck by removing the neck's plastic extension tube.  I did this, and immediately thereafter was rewarded with my first-ever overflow of a significant amount of fuel out of the tank vent hose onto the ground after a fill-up (cooler fuel from in-ground tank expanding as it absorbed heat from the adjacent engine and exhaust on a summer day).  Fortunately for me, my GL1800 neck removal was reversible.

I'm not saying that modifying the filler neck should never be done, but if modified it should be done with the awareness of the potential problems (i.e., not done in a manner which eliminates virtually all of the air space "cushion" the designers provided in the tank), and the bike then ridden with awareness of the mod (e.g., on days when the heat of the day could lead to excessive fuel expansion, not filling the tank to the max possible).  In my case, I still modified the re-installed Gold Wing's filler neck a bit, but did so in a manner which still preserved some of the air cushion.