Sure, that can be an advantage; it's definitely worth something.
However, at +3 Virtue points/ability, the cost is mostly too steep, in my view.
This is particularly true considering that other Realm traditions make it harder to learn Supernatural Abilities, and on top of that often those Abilities cost only a minor Virtue.
So yes, your initial proposal to knock down Powers to Minor might be reasonable (I suspect keeping all Major was meant as a balancing act against the Divine, given that the infernal needs only 2 + 6 rather than 3 + 7 Methods and Powers).
However, let me also call your attention to one oft ignored character generation option: Infernal Blessing (RoP:I, p.85). At the cost of 3 Warping points and 1 xp in an Infernal reputation, as a Major Virtue it effectively allows you to take two Major Virtues (Infernal Greatness, RoP:I, p.86) that stay with you as long as you are not rid of sin. It is deeply unfortunate that it is tainted, and only half of your Virtue points at most may be Tainted...
... but really, embrace the mood and tempt your players into the ultimate sin of munchkinism. If they want an Infernalist starting with all Methods and Powers, subtly suggest they might do the following. Pick Infernal Blessing, once. With the two Major Virtues it yields, subject to SG approval, pick Infernal Blessing twice. With the four resulting Major Virtues, pick Infernal Blessing four more times. If the SG approves, as she will undoubtedly do subject to some real-life bribe, the resulting eight Major Virtues can be the 2 Infernal Methods and the 6 Infernal Powers - which are not tainted, and thus balance out all those Infernal Blessings!
Diabolic!
(yes, yes, they now have a total of 21 Infernal Warping points, i.e. a Warping Score of 2(6), and thus a Minor "free" Flaw from Warping .. but that's a small price to pay, isn't it? And they also get 7xp to an appropriate Infernal Reputation, such as Munchkin 1(2).)