When using small diamater batteries, one needs to make sure the battery can supply the current the application needs. The PowerSonic SLA can output a max of 54 amps. The trusfires have a capacity of C = 2.5ah ... if their current output is, as an example, 20C, the the current output would be 20 * 2.5 = 50 amps.
I guess your friend has tried this already, but just more due diligence before spending you money.
Here is an example of what radio controlled race car hobbyists used ... note this has a hard shell case, not like the soft (and easily damaged) cases the model airplane guys used: