I'm currently using 4x 63V 470uF and 2x 100V 220uF Silmic IIs. Higher voltage versions sound better than 16V versions. However those are way too large to fit without removing the battery & top half of the stock case. 16V 22uF Silmic IIs are gauranteed to fit every position with the stock battery/case intact, but there's room to replace 3 of the 6 stock electrolytics with 47uF 16V versions instead if you bend the leads on two of them slightly. If you're really creative you may be able to fit larger versions.
I bought the 5,500 mAh battery on Ebay. With the increased power draw from the Burson V6s and the much larger caps it provides around 5.5 hours of juice. I also bought a 10,000 mAh version but its double the weight and much larger, so I'd recommend 5,500 mAh because its the same size & shape as the stock B4-X and sits underneath nicely:
https://www.ebay.com.au/itm/High-Qu...028564?hash=item263d8735d4:g:i5YAAOSwb5VfxGCq
Here's how it looks, very much "in progress". The small circuit at the back is a Belleson 12V super regulator. My soldering skills won't be winning awards anytime soon.