There are in reality 2 things that you have to consider; the Amp rating of your battery is one - and it is recommended that you don't thrash it to much more than 70% of its ability, and the rating of the spring before it collapses.
The spring will not cope with coils of about 0.4Ω, the evidence is that they slowly sag. I'd suggest that since we are only a few weeks away from the release of the new kits that modifying your Reo just now is possibly counterproductive!
Stacking batteries is only done in regulated mods that have electronics and typically only in matched pairs - they are paired for life - used together, charged together, because any mismatch in them stresses one battery more than the other.
You also have to remember that stacking batteries only increases the voltage - you are still limited by the Amp limit of any one battery.
T