Actually, I was not referring to the parasitic drain while off - I was referring to the drain while on. Compare the V20A with the M20A, for example, both use the R5 emitter in a 2xAA format. The V's max runtime is stated as 1 lumen for 100 hrs while the M's is 4 lumens for 200 hrs. This difference seems to evident across the entire line - variable vs manual.
The high-end is not materially effected, however, which makes sense. If, for example, the parasitic draw of the magnetic ring is around 10-20ma, then it be immaterial in relation to high output modes which will draw around 500+ma. However, the low low modes of 1 to 4 lumens draw only around 3-10ma, and so the parasitic draw of the ring becomes very material for the low-end runtimes.
Bottom line is that, if you use your lights on the higher end a lot, the magnetic ring will not make much difference. However, if you use your lights on the lower end a lot, then it will probably be better to get a manual clicky...... or just carry spares and change batts more frequently.