Do what thou wilt shall be the whole of the Law.
I question this approach. I understand the thread of thinking that led Yorke in that direction, but I find it suspect.
Osiris, before his death, was meant to signify specifically natural humanity - particularly the maturing Ruach, but then (as a transcendental symbol) the evolution through which that passes. Horus already represents "the next generation" of ourselves and is already a transcendental symbol, a "stand-in" for the HGA, etc. His age (in his various incarnations) is a symbol of the maturity of that awakening in us.
Nonetheless, since the question was raised...
I have no problem with RHK as "perfected" (1) because I see the Thelemic forms of the Horus archetype already having that quality and (2) because he has matured, i.e., is the seed-idea newborn in Harpocrates having come to maturity. OTOH, sure, Heru-Ra-Ha is more abstracted and therefore "higher." Even more, we can turn to Hrumachis as a form so transcendental that it is used as a symbol of that which spans all the aeons, being a (currently useful) token of each "new dawn" that breaks upon humanity.
I agree with you on your root interpretation, I think, but will paraphrase it to make sure: Osiris is the symbol of the cross, the 4. Horus is the symbol of the Pentagram, the 5. Osiris in life is the Elemental Cycle of the First Order of the traditional Order, where one is taken apart into pieces and relies on the Portal to reintegrate them. Isis then pulls off the magick that completes him - adds "his secret 5th part" - and in his post mortem formulations he has become perfected by the addition of the 5th. OTOH, Horus already fulfills that - in Temple of Thelema we sacramentally show Horus, the 5, as that which completes and thereby transcends the 4-ness of Osiris - and, therefore, is the natural next step.
Compare the original formulations in the early A.'.A.'. grades: The 1=10 (Malkuth) initiation is of the Four Elements and the highest ideal is a form of Osiris. The 2=9 (Yesod) initiation adds something from beyond that framework and introduces Horus as the new high ideal, specifically manifested as separate aspects at the four quarters which are integrated in Ra-Hoor-Khuit as the central fifth about which they are placed.
FWIW.