I would strongly agree that the Alchemical Greater Elixir is one of the other excellent alterntives to Becoming Fay. Provided the individual pursuing Alchemical "immortality" uses Inscription of the Soul to adapt his or her Spirit into a Talisman. Otherwise, the risk of losing all of the achievements made after using the Greater Elixir would be too great.
Perversely, studying Diedne Magic before undergoing Transformation of the Body would offset any loss in Spontaneous Magic. I say perversely, because the Dark Secret would count as an interesting Ordeal that could be a cause for later friction within the House. At the same time, it would challenge the (to me, largely false) accusations that the Diedne were Diabolists because the Magus would be Magic and Faerie. I realise that it's a subject for another thread, but I can't see Diedne Magic being entirely forbidden within the Order, if only because Bonisagus Magi surely studied it and continue to pass down those techniques for stronger Spontaneous Magic.
One needs to keep in mind that a Magus who has Become Fay retains the ability to undergo further Mysteries. This is not possible for any of the other types of "immortal". They could pursue esoteric studies granting them greater and greater mastery of their Arts, of Magic, and of Faerie. They also do not suffer from the need to alternate every single season of learning with an investment of Vis in their Talisman in order to retain what they have learned. When it comes to completing major projects, Fay Magi are in a far better position than Alchemical "immortals". They can continue to learn within the parameters of their extant Abilities, and the above-mentioned power to gain through further Mysteries could represent an alternative method of adding Abilities even once they've undergone Transformation of the Mind.
I'm curious as to whether Alchemical or other "immortals" can engage in original research (i.e. pursue Hermetic Breakthroughs), since Stabilisation of a discovery requires an extra season, which, according to the extant rules, might result in a loss of the discovery (I'd be inclined to let them "lose" the previous spell, relearning it later from their notes, saving the Stabilisation instead). As is explicitly stated, Fay Magi can perform original research. Immune as they are to Warping, they could conceivable surpass many of the Hermetic Limits and even their own within a few decades to centuries.
Which raises the question of why they aren't more prominent within the Order. With their extremely long careers, they could conceivably wind up dominating many fields.
On a related note, does any suppose that Caitlin Suil Uaine, the "Half-Fay" leader of the Crun Clach Covenant in the Loch Leglean Tribunal, is the beneficiary of at least partial Becoming? As she was presented, she broke several of the existing rules by integrating the Faerie Companion and Magus types. In 5th Ed., she might be thought of as a Maga who was born with Strong Faerie Blood who subsequently underwent Transformation of the Body and possibly Spirit. She doesn't have a listed Might Score, obviously. If she did, it would probably be enormous. Well over 100 (which is attainable with enough assistance).