Most 1911's come with a ~21 lb mainspring from the factory. It requires a pretty good bit of momentum to bring the hammer back, which still has to happen with a conversion top-end. A lot of guys are having trouble with the conversion units functioning well.
My two cents.
Actually....16 lbs is the standard factory rating....per the slip that came with the 16 lb Wolff recoil spring I have sitting on my desk right now, as I look for anything to take my mind off of the calculus final I'm supposed to be studying for. Not that it really matters, I'm just saying ha.