In Super Mario Bros Z, one of Sonic's (And Mecha Sonic after he copied it) abilities is the light speed dash, used in Episode 7 while fighting Mecha along with Mario. The technique, though never officially named in the series itself, amplifies the hogs' speed to the point that Mario appears to be completely frozen.
For this we'll use this formula: (Reference object actual speed/reference object apparent speed) * apparent speed = actual speed
Previously, Mario could outrun the Koopa Bros' special attack, which was traveling so fast that it was leaving a trail of flames in its wake, thus placing Mario's movement speed as >baseline Hypersonic, or mach 5/1715 meters/second, Sonic and Mecha are both faster than Mario to an unknown degree, and their apparent speed doesn't change after using the technique, Mario is just now frozen from their perspective. Thus for his apparent speed I'll use the speed of a snail. That should be pretty easy right? Just look up the average speed of a snail, plug it in and boom! We're done, right?
Well, as it turns out, the speed of a snail heavily varies, ranging from as "high" as 0.013 meters/second, to as low as 0.000023 meters/second, with a whole slew of numbers in between.
As such I'll do a low end using 0.013 meters/second, a mid-end using 1 millimeter/second or 0.001 meters/second, and a high end using 0.000023 meters/second
LE: (1715/0.013) * 1715 = 226,248,076.923 meters/second, 0.75468 C, Relativistic+
ME: (1715/0.001) * 1715 = 2,941,225,000 meters/second, 9.81087 C, FTL (Given how this isn't even 2% less than baseline FTL+ and how Mario upscales from baseline Hypersonic and Sonic and TMS upscale even further from him, I'd just consider this one FTL+)
HE: (1715/0.000023) * 1715 = 127,879,347,826 meters/second, 426.55959 C, Massively FTL
Well that's a little faster than just light speed.
As for who this'd scale to, obviously Sonic and Mecha with their dash would scale since they're the ones performing the feat in the first place, however I'd argue the base, unamped cast should scale to some degree as well. Mario, while appearing frozen from their perspective, could fully react to and dodge multiple of their clashes, thus they should at least get MFTL Reaction/Combat speed. This would mean Cape Mario's movement speed also scales, since he could blitz Bowser (Who could keep pace with base Mario) so badly he appeared to be teleporting, likewise Semi-Super Mecha would also fully scale, since he could blitz/nearly blitz the Axem Rangers X and the heroes while only toying with them. Base Mecha's own reaction/combat speed would upscale, since he regularly stomps the main cast and could fully keep up with Cape Mario, who can normally blitz characters with this level of reaction speed.
As Psycho pointed out on the FC/OC wiki, this is probably calc-stacking, and as such, due to the move essentially being the exact same as its in-game counterpart, I'm just going to list the technique as being Speed of Light. Base characters' reaction/combat speed would still scale, meanwhile characters with power-ups would be FTL with them (Since power-ups make them faster and by definition Speed of Light is a single value and being any faster would then boost you into FTL) with Turbo Mecha Sonic's reaction/combat speed scaling to FTL in base, since he could keep up with Mario's cape form, which can blitz enemies comparable to Mario's fire form, which itself is faster than his base form. Semi-Super Mecha Sonic and Sonic and Mario's super star form would be FTL all around, with the former blitzing the Axem Rangers X (Comparable to the base cast) and the heroes' super forms blitzing Mecha Sonic's Semi-Super form.