So the V5i arrived today from Burson. Firstly, forget about fitting it in the E12DIY casing (in case that wasn't clear from other people's comments). Here's a pic to demonstrate:
https://photos.app.goo.gl/76ctI1r2gLwUTqrt1
Sadly, my first test session resulted in fried HA5002 buffers!!!! I should clarify that this was NOT user error as the V5i worked perfectly with the LM49600, but was a bit shaky with the BUF634. I'm not sure exactly what is going on to cause the problems (hissing, etc.), but I do know that my HA5002 are junk now.
I expect to hear wonderful things from the V5i once I'm brave enough (and my ears have settled down enough) to try it again, but for now wanted to warn you good folks to be careful if using the V5i op-amp in your E12DIY.