Grimoire is described as an enhancement on page 149 of the index from GURPS Supers, while page 150 refers to it as a modifier.
Pg 110 "Comic Book Wizardry" makes reference to it, with it's central write-up on page 27.
27 clarifies it actually isn't a modifier at all, but rather a VARIANT of Modular Abilities introduced in the book, along with Reconfigurable Mechanisms and Spirit Trapping.
Rather than modifiers, MA variants introduce a "cost per slot" which has a base cost and a "cost per point".
B71 uses the phrase "types" when defining the first batch of variants, including:
- Computer Brain
- Chip Slots
- Super-Memorization
- Cosmic Power
Given there is no baseline version of Modular Abilities, it seems misleading to call any of these versions/types/variants as a enhancement or limitation explicitly, with no point of reference. All the other cheaper variants might be seem as 'Limitations' in respect to Cosmic Power, for example, but Cosmic Power could also be seen as an enhancement to any of the other cheaper abilities.
Grimoire is especially weird to think of as an enhancement (despite the index calling it that) since it is among the cheapest forms of Modular Abilities.
As such, P150's "Modifier" description of it is probably more appropriate, since 'modifier' doesn't define it as either enhancing/limiting and even includes things which are neither (0% features).
Grimoire has the same 5/3 slot pricing as CS/SM.
The problem with Grimoire is that it has a vague "taking time to review the sigil" requirement when consulting the book.
Chip Slots had the requirement of 3 seconds to insert/remove physical chips, while Super-Memorization required 0 seconds to forget a skill and 1 second per character point to memorize them.
Super-Memorization was a faster way than CS to learn abilities costing 1 or 2 points (same or more time to learn stuff costing 3+ CP) but it's primary difference was the different mechanism of losing abilities:
To force a Chip Slot to lost an ability would (if you could not convince / mind control them into removing a chip themself) forcibly removing a chip from their socket: something with unknown combat mechanics.
Super-Memorization could not be removed that way (equipment was only needed during the longer learning times, but not to retain the mental advantages) however it might be possible that it could be more vulnerable to countermeasures (ie a psychic mind wipe might reset super-memorization to forget via amnesia, while Chip Slot might be paired with tech modifiers making it immune to biological-affecting abilities)
Super-Memorization was also better for rapidly changing between low-cost abilities. It would take at least 6 seconds to change Chip Slots and in 6 seconds someone with Super-Memorization could instantly forget 5CP of ability and replace it with a different 5CP of ability.
Chip Slots cycling might take more than 6 seconds if GM added time to retrieve a new chip slot from a pack, if it wasn't in hand, such as 1-handed changing instead of 2-handed... and while DROPPING is a free action, it might take a maneuver to safely store a removed chip slot into a pack. However these considerations could equally be used to bloat the cycling time of Super-Memorization requirements (book/film/tape) unless they had a special AV setup like a movie theatre, and even then they would need a means of cycling through available combinations to find the one they needed, prior to doing the 1s/1CP memorization process. Time needed to flip through pages for required spell is never dwelt upon, and might depend on what variety of options is available.
That might however be represented as 'Requires Preparation', with the assumption being without RP that you'll always automatically flip to the "right page" of the grimoire.
B71 explicitly mentions "from a grimoire" in the Spells Only limitation for Modular Abilities, likely intending it for the Super-Memorization variant of it, which was the only one of the basic set forms that fit the description.
S27's same-priced type using bound sigils (which might be stored in a Database) is explicitly "like spirit-trapping, but you don't have direct access".
Spirit Trapping costs MORE points (6/4 like Computer Brain, which has a suggested transfer rate of 1 character point per second, like Super-Memorization), and requires entering a vague "several minutes" trance.
Probably the reason ST costs more than Grimoire is due to being able to swap abilities without needing equipment, whereas Grimoire requires physical equipment to make the skill swaps. This is very similar to the contrast between Computer Brain and Chip Slots in basic set.
Contrasting Grimoire to Super-Memorization is more difficult: in both cases you have a requirement to consult a book to switch your skills.