The majority of the information below is obtained from the following two sources:
Kz3r0's post on rpgcodex.net's forums
and
Steven W. Carter's Arcanum Guide and Walkthrough on GameFAQs.com

Jayna Stiles is able to produce up to 4th level Therapeutics for boosts, which includes the Elixir of Persuasion (+2 of 20 to Persuasion), the Elixir of Physical Prowess (+2 Strength), the Liquid of Awareness (+2 Perception), and the Tonic of Increased Reflexes (+2 Dexterity.)
This forum post (here) indicates that most of these last for 8 in-game hours.
She also learns to produce the remarkable Healing Salve (base 20 HP restored), the lovely Fatigue Restorer (base 20 FP), the Poison Cure (reduces poison by 80 points), and the Fatigue Limiter, which purportedly halves Fatigue consumption.

Magnus is able to produce:
the Spike Trap, which deals 15-25 points of damage by her accounts and bypasses Armor Class;
the Trap Springer, which disarms trapped containers but does not open locks nor does it work on floor traps;
the Auto Skeleton Key, which improves lockpicking by 10% for those not antagonistic to technology;
and Eye Gear, which raises Perception by 2 Points. Possible to find Eye Gear without bringing Magnus along, a kindness to evil characters unable to recruit him.
Pure Ore, in itself relatively useless;
the Balanced Sword, lauded as the most godawful powerful weapon in the game;
the Feather Weight Axe, crucial to a few found schematics;
and the Dwarven Gauntlets, which increase damage when fighting unarmed and also act as armor.

It sounds like Jayna's more important than Magnus, between the two.

Sadly, if you want the Goggled Helmet (possible +3 Perception) from its found schematic, you'll have to learn Smithy yourself. Magnus doesn't become proficient enough to produce the Helmet of Vision, a level 5 device which is a prerequisite item for the Goggled Helmet.
Of course, at that point you may as well just put a point into Perception, I suppose, unless you've decided to make armor for yourself.
Given some of the late-game armors available through advanced Smithy, that's honestly not that bad an idea for a technologist. I found them rather enjoyable. The Machined Platemail was amusing, especially because it required no actual expertise in the Mechanic discipline, only an almost absurd weight in Manuals.

If the spell works, it will always give +4 DEX, since the power of those elemental stat boost spells is fixed.

Your problems will come when your Tech Expertise gets so high that the spell will fail to cast on you - tech expertise doubles as a kind of magic resistance that affects good and bad spells.

You'll cast the spell fine, but it won't 'hit'.