These are very good batteries as long as you do not want to sub-ohm on the SVD. If you plan to stay at 1ohm or higher, buy these. They are only good for something like 7 amps. 20watts @.5ohm is going to be 6.8amps or so. For sub-ohm, you should go with something like the Samsung 25R or the purple Efest. You could even get away with using the Panasonic 18650PF (10amps).
The manual sort of states that 6 amps is the max?
Well... if you consider that the 6A is the output max, and there are efficiency losses related to the circuitry required to regulate the output... and given the min input is 3.2V... I would figure the input amperage could be pretty close to 8A @ 3.2V max. While that is only 15% over spec, an edge case and only when the battery is really really low and the output settings are at the max... I would still suggest a battery in the 8A+ constant current range. I bet there are lots and lots of people who have been using under-rated batteries since the first day it came out though without any incident.
Teacher always said show your work...
20W divided by 3.2V input means at least 6.25A battery draw at 100% efficiency. Since this is an extreme case and efficiency isn't 100% (reality + regulated), let's call the efficiency 80%... 6.25*(1/.80) ~ 7.8125A input at 3.2V. Sanity check... 3.2*7.8125 -> 25W and 25W input * 80% efficiency -> 20W output... yup, sane.
NOTE: Efficiency could be upwards of 95%, which would only be <7A input...
I have seen the constant current max for these batteries listed from as low as 3.4A up to as high as 15A CCR. None of the Panasonic datasheets I looked at had a CCR listed, but the battery dropped output voltage pretty fast and low at 6.8A draw either way.
Simply put, it's a fine battery with decent output capacity even under strain, but if you are drawing 6A+ from this battery... the output voltage will suffer.