The frequency of Sperry tests has little positive effect on track speed. The issues that effect track class, and therefore speed, are related to over all track geometry. And by that I mean everything that has been mentioned here before, weight/type of rail, ties, surface, sub-base, bridges, culverts, and so forth. The mandated frequency of Sperry testing is primarily a function of amassed tonnage for a certain section of line, unless the host RR feels that they can justify the additional expense to reduce service failures of the rail, or in specific cases the increase over the mandatory minimum is mandated by the FRA. One such example of FRA mandate is the Portal Bridge on the NEC, it had been under a 90 day testing cycle when I worked at Sperry due to the style of miter rails, they had failed and caused a rather nasty wreck and as a consequence were tested at a higher frequency than other bridges with different miter rails.
Magnetic induction is not a new technology, it is in fact the original method perfected by Dr. Sperry around 1925. See this page from the SRS website
for a history of the development of internal rail inspection. What is new is the miniaturization of the components in the system, so much so that now instead of having to use the rail bound cars high rail trucks are utilized. The newest detection method is with laser light , but to my knowledge it hasn't progressed past the lab for rail-flaw detection. The newest real-world development has been non-stop high speed ultrasonic inspection. UT inspection was limited to about 13 MPH, before the development of a new style of wheel that could catch the signal bounce-back quicker and a much improved artificial intelligence for the flaw detection software. Now they can test up to 50 MPH.
And Sperry charges by the hour, unless you have a contract.