It would behoove us to make sure that we understand what all parties are trying to communicate before we say anything ourselves, for we shall surely only frustrate each other further if we do not understand each other. Allow me to summarize the arguement of those who do not agree with the upgrades:
"Dante and Mundus are MFTL+ due to a feat in a cutscene. However that feat must be an outlier, for we see them move at much slower speeds in a subsequent cutscene. Furthermore, base Dante fights Mundus in the second phase of the fight, and since base Dante is obviously not FTL, this surely proves that the MFTL+ feat is an outlier. Lastly, DMC2 is rather lacking in such speed feats, and in that game, Dante supposedly exceeded Sparda in all regards."
Now, my rebuttal: Firstly, the validity of cutscenes supercedes the validity of gameplay in almost all cases. This is something we must consider in all games. Therefore, an impressive showing in a cutscene supercedes an unimpressive showing in gameplay. And as far as unimpressive showings go, two opponents flying throughout a planet's atmosphere and firing lightning, lasers, meteors, and flaming dragons (!) at each other while doing so ain't half bad. Besides, it was essential to the level design that the fight take place near the surface of a planet (for the second phase), and how in the world would the gameplay have conveyed MFTL+ flight under such circumstances without dizzying the player?
Now, the matter of base Dante in the second phase of the fight must be addressed. However, those who took the time to read my blogs before making claims know that Devil Arms can increase the users stats significantly, and vice versa. In fact, Dante wields the Sword of Sparda for a good period of time in the game, but it is only at the final battle that he can activate that specific weapon's specific Devil Trigger. Couple that with the fact that Dante could react to and deflect an attack from Mundus after failing to do so before his eyes obtained a red glow (a sure sign of obtaining his latent power) and it is evident to the observant thinker that even Dante's base form had obtained much power at that point. In the third phase, when Dante had neither the Sword of Sparda nor the Perfect Amulet, a Mundus who had just been dissolved and had a universe collapse on him laughed off Dante's attacks.
So, about DMC2. It is true that Dante does not have many speeds feats aside from the usual Sub-Rel. to Rel. stuff we've come to expect from base form. The difference is, he's got nothing to worry about by not going 100% here. The only time Dante should have speedblitzed and didn't was against his first bout with Arius, and that was in a delicate hostage situation that overpowering and unsettling Arius could have worsened (if the scene wasn't just PIS outright).
EDIT: Aw, man, that showed up larger than I thought I would. Sorry for any inconvenience.