I checked the manga from the saiyan saga to the end of the Frieza saga. And and as you said, he indeed lowballed Goku's speed by ignoring zenkais and blitzed (because he doesn't know how much it increase his speed).
But:
He used base form Goku when he first arrived on namek (power level 90 000 in base form). And then he used SSJ (saying it would multiply speed by 40 (most likely 50) considering Goku kaioken 20 had the same level than 50% Frieza).
With Goku having a power level of 90 000 he said he'd be 28 times faster than light as SSJ (according to his calc, base goku is at 70% SOL).
But while I checked the manga, I noticed something interesting. The higher your power level is, the faster you are. It was said officially in the manga as you can see here:
http://z.mhcdn.net/store/manga/214/..._ball_-_v017c249_-_page_006.jpg?v=11382940184
http://z.mhcdn.net/store/manga/214/..._ball_-_v017c249_-_page_008.jpg?v=11382940184
Vegeta himself said it "if my power level improved, then obviously my speed has improved as well".
Also, have you saw what he did to Kiwi (power level 18 000) while he had himself a power level of (24 000)?
He literally was fast enough to react to an attack from behind and dodge it, and Kiwi even couldn't see him at all. And yet the difference between them isn't that big (24 000/18 000 = 1,33333 etc ...).
But during his fight against Frieza, Goku's power level (base form was 3 000 000 instead of 90 000).
Goku's power level was 33,33333 etc ... times stronger than it was when he just arrived on namek. So Considering Kaioken 2 just double your power level but make you twice faster, and considering we never saw someone having a huge power level but being slow (the only exception being "Super Trunks" when he fought perfect Cell, and perfect Cell doing the same mistake against Teen Gohan SSJ2)
... So we will be VERY nice and say it multiplied his speed by "only" 10. That makes him 280 times FTL instead of 28 (as SSJ during his fight against Frieza). Making both Frieza (50 % or 100% whatever) and Gokus SSJ MFTL.
Even if we increase this only by 4 (instead of more than 30) Goku SSJ is still MFTL (28 X 4 = 112).
But even the "10 time" hypothesis is pretty low compared to 30 ...
However, even if we do not count trainings for example, as a SSJ2 then Goku would be 560 FTL (twice faster than SSJ).
And as SSJ3 he would be 2240 time FTL (4 times faster than SSJ2).
And 10 time is still very low compared to 30. And we haven't count training (such as the training in theh hyperbolic time chamber for example).