Spool = fully locked diff (no diff action). So yes, spool is the same as saying locked diff. It would widen turning radius (make the M05 chassis understeer more) as it splits the torque 50/50 all the time to both front wheels. Would cause a little more scrubbing of the tires during cornering as well, which may (or may not) have a minor effect on life of the tires. But it all comes down to setup of everything else in its entirety as well...

I wonder if that diff is made in the same factory as the 3racing one - the plastic and general forming of the parts look identical (but I'm also comparing the pic to the 3racing one's I have on hand). If it is, I wonder why they opted for a plastic spider gear cross as opposed to the split steel one... (not that it truly matters).