First of all I wouldn't get a smaller motor just yet-you can detune the system for long finals more easily/efficiently using the hobbywing program box (and adjust other things as well):
http://www.ebay.com/itm/HOBBYWING-RC...item1e70f588a7
I'd say a 2200kv motor is about right to start with, it's what I had in my HPI Vorza when it was new and that felt good to me at the time. This leads onto the subject of gearing, i've looked at the XB808e specs and fortunately we can compare it directly to Vorza gearing.
The 808e and Vorza share the same crown wheel/pinion ratio of 43/10 (inside the front/rear gearbox cases) which makes things a lot easier. Standard Vorza spur/motor pinion ratio is 48/15 which as I say felt about right on a big track (South Coast RC/Clanfield) when the car was new and had the 2200kv motor. 48 divided by 15 = 3.2:1 so you're looking for a motor pinion which gives you the same ratio with the XB808e's standard (42T) spur gear.
42/13 = 3.23:1 (slightly lower than std Vorza giving better acceleration/runtime and lower top speed) and 42/14 = 3:1 (slightly higher than std Vorza giving slower acceleration/less runtime with higher top speed) so i'd start off by getting 13T and 14T pinions and see how you get on.
Regarding batteries, unbelievably these are in stock in the UK and are actually good value for money:
http://www.hobbyking.com/hobbyking/s...arehouse_.html
I've been using them for over a year now, they're a bit temperamental when balance charging due to being 2S2P but have more punch than Mike Tyson
, just remember you'll need 2 of these wired in series to make a 4S pack. I now have a 150A Hobbywing/2700kv motor combo and 48/17 gearing and they usually give me 17-18mins runtime with the LiPo cutoff set to 3.2v so with your 2200kv they should last a 20min final if you drive carefully/play around with the program box (the track I race at only runs 5min finals).
Or there's these which I haven't tried, 6600mah could give you that bit extra runtime you need:
http://www.hobbyking.com/hobbyking/s...arehouse_.html
One other tip for 1/8th electric, see if you can double up the front shock tower (fit 2 back-to-back). Because they're heavier than nitro buggies these tend to take a hammering and until I did this I was going through a shock tower a week-literally
. 6 months after doing this mod I haven't bent a single one! Don't worry about the weight gain, I don't know about the 808e but a Vorza front tower weighs 27 grams which is pretty insignificant when the all-up weight of the car is about 4kg. Hope this helps.