You are asking is it worth speeding up? But you focused only on the savings! Remember the actual reasons behind 250kts below 100!!
Bird strikes result in less damage at the slower speed.
If everyone is doing 300 kts ATC are more likely to miss their mistakes and ours.
Was the rule not originally brought in following a mid-air collision or a few near miss!?
I am interested to see a figure for the fuel savings though, i suspect they must be very small as the slower speed penalty may be countered by the higher ROC to higher levels.