Okay, at these numbers I ran some calculations to see if things need tweaking and to determine base values for the enchantment guidelines. I ballparked a reasonable "Novice Architect" score, including scores reasonable for an unspecialized magus a few years past gauntlet who could have access to RHA. I also generated potential scores for a "Master Architect", who has a reasonably specialized laboratory and dramatically higher scores, as well as more bonuses (such as a maxed out numerologist's book and an apprentice).
According to my estimates, the Novice will have a score of ~17 in the Devise step, which means that he could affect a standard size 8 aura of up to strength 3. A Master level Architect would have a score of at least twice that, which means an aura score of 20 at size 8. I believe I'm going to add a factor including the number/depth of regios in the target area, to give the overflow score some value (and further limit weaker Architects).
In the Invest Step, the Novice and Master diverge, as they now add their arts scores. Novice has ~35, the Master has a lab total of 90. With those numbers and the existing guidelines in the Mysteries and RoP:M I generated this first draft of the mystery spell guidelines:
Creo Vim: Create/Strengthen Aura Tether by 1 (Total 30+5 per maximum pret. tether in aura)
Muto Vim: Double of Halve size of aura/Regio (Base 4) (Min. 20 for boundary)
Rego Vim: Gate between regio levels (Base 15) (Min. 35)
Muto Vim: Tint Aura (Base 5*Level) (Min 25 for boundary)
Muto Vim: Aspect Aura (Base: Aura*10+5) (Min 25 for boundary)
With an invent score of 35, the novice cannot create Aura Tethers, but he can tint and aspect auras, particularly if he is not trying to affect the full aura. There are several useful effects he can accomplish, from giving instilling personality traits in covenfolk to boosting particular types of labwork via aspecting, or toying with regio boundaries. There's enough there that I think the guidelines are a success in that respect - you don't need to be a crazy nutter superpowerful magus to use the virtue.
The other stumbling point with HA was the hypothetical "Super powerful magus attempting to boost an aura". It was just so... piddly compared to the extreme effort and cost required. With these guidelines, the highest level effect is creating a strength 5 preternatural tether, which has a total of 55, easily within reach. This means that the Master can pretty easily boost an aura by +5 by serially boosting an edifice's tether score +1 at a time, but only if the aura isn't capped out on preternatural tethers already. I think that limit really caps how powerful this can be, without needing to get into the crazytown high base levels required in HA.
The real project of a mad Master magus in RHA isn't empowering auras (which ultimately isn't that strong), but in aspecting them, or parts of them. Raising an aura by 1 gives +1 bonus. Aspecting an aura gives you the full strength of the aura as a bonus, but to more limited activity. The costs for that can easily get up to high levels/costs, and are of more limited general utility.
All that needs work at this point is revisiting and revising the exact numbers, and smoothing out the rules. Aspecting and Tethering will need some cooldown/windup rules, for the setting if nothing else. Rapidly boosting or twisting an aura will shatter it, creating Regiones or worse, as described in RoP:M. Outside of the bounds of the mystery, for sure. I think it's easier than I thought it would be to allow regular, non-mystery enchanting guidelines using the same rules, too.