Hopefully this thread will help to make our knowledge of using BB kits on the newer EFI bikes readily available.
Firstly, here is a copy of Bill Blues' post on another thread on 11/15/2019:

Bill's Website: Home Page

It seems a "nobrainer" that a 351 EFI with the Barker system and a lidless airbox would make well over 30hp, once Bill's EJK is reprogrammed to fuel it ! Very exciting - but someone will need to do the Dyno leg work to create the new EJK fueling for such a bike..

 

I recently fitted Bill Blue's 351 kit to my KLX 250S EFI bike, with EJK controller provided with the Bill Blue kit, and for everyone's benefit I can share my sad experience with it: bike hesitates to startup which is one problem, but even bigger problem is that reving up to 6-8k rpms and releasing throttle it dies out. My mechanic and I spent 3 hours trying out all sorts of EJK settings, but no chances of resolving this problem. I contacted Bill, he was quick to pick up the phone, but couldn't help me on this, saying he's not a fan of EFI (what kind of answer is that for someone distributing EFI kits?). Contacted EJK asking for recommended settings on the controller - no response. I tried raising rpms, taking airbox lid out, unplugging stock exhaust to see if any of it helps to get rid of stalling issue - nothing helps. Having spent ~1000 eur on this upgrade (I'm in Europe, so customs taxes are applicable), and 300 more for mechanic work, I just wanna cry for being so naive! For those of you thinking of fitting 351 kit on KLX250 EFI, consider other options out there.. perhaps 292cc upgrade might work better, I hope.