Can't comment on the difference between 50' and 100' as am only a humble SAR boy... but do think it should be pointed out that this new system with the amount of time it takes to sort out a low level booking now, does seem to encourage people to go up in the sort of areas ( above 500ft, below the zero degree isotherm when like me you've got bugger all icing clearance) where your a lot more likely to bump into a lot more traffic (our fast jet brethren for a start)
Once again just a thought, and of course on actual SAR ops all low level bookings can be done retrospectively.