Lots of things in SA are much more overpowered then this mod

I'm still processing that foundries more then double amount of resources (and this is without modules!!!) you get from ore and here you say that bigger and faster trains are overpowered

From my (albeit minimal) testing, the trains would get extremely quick, moving a vast amount of resources. I suppose for longer play processing millions of resources every minute it could be considered balanced, but for casual play it's quite powerful, at least in my opinion.

You could change the defaults to be less extreme - maybe copying the acceleration/speed bonuses of quality rocket fuel? (of course stacking with the effects of said fuel)

I'd suggest changing the speed formula to wagon.max_speed = wagon.max_speed * (1 + (qvalue-1) * 0.3) - this results in legendary locomotives using coal having the same speed as normal locomotives using rocket fuel.
Alternatively, the *0.3 could be changed to *0.15 so a legendary train with normal rocket fuel has approx. the same top speed as a normal train with legendary rocket fuel.

Currently, quality locomotives use the same amount of fuel per distance ignoring quality level (2.5x speed, but also 2.5x consumption from higher burn rate). Since with the nerfed speeds, this would result in worse distance efficiency than normal locomotives, I'd suggest giving quality locomotives improved fuel economy by keeping the burn time of a certain fuel constant.
To do this, add if train.energy_source.type = "burner" then train.energy_source.effectivity = (train.energy_source.effectivity or 1) * qvalue end to the locomotive changes (this cancels out the higher acceleration, resulting in normal fuel use of 600kW).

With legendary rocket fuel, legendary locomotives reach speeds of 420 (0.15 factor) or 485 (0.3 factor), which feels somewhat in line with the 356 vanilla legendary RF speed (vs 259 coal speed and 298 normal RF speed).
With normal rocket fuel, legendary locomotives reach 352/406 kph - the first definitely seems relatively fair, the latter is quite strong. Legendary locomotives cover 18%/36% more distance with the same amount of fuel.

Perhaps you could add a "speed OPness" setting with 4 choices: "vanilla-ish" (0.05), "balanced" (0.15), "strong" (0.3), "OP" (1 - ie. the current behavior)...
(vanilla-ish would result in only 1.5% speed per quality level, ie. legendary with 279 with coal, 320 with normal RF and 367 with legendary RF)

"conservative" (0.1) and "powerful" (0.5) could also be added to the list...