Thomson probably had the potential to bowl 100mph but I highly doubt that he often did. I think, judging from watching some of the current bowlers and watching him on youtube that he mainly bowled inbetween or around 85-90mph.
In Dennis Lillee's book, Thomson, Lillee, Roberts and Holding got their speeds measured in a match and Thomson's was over 150kph, IIRC. However, I will try to find the book and get the exact figure.