The upcoming ACKS Heroic Fantasy Handbook uses a revised approach for spellcasting progressions using a "discounted spell level" calculation. As Alex and others have noted, this suggests that the "partial Arcane" progressions (such as the Nighblade, Elven Courtier, and Gnome Trickster classes) are being shortchanged in terms of casting power.
This is the subject a recent Axioms article (Lesser Arcane Value Progressions). In it, Matt and Alex propose revised spell progressions which match the expected 1/3, 1/2, 2/3 power levels.
You could take a different approach, though...
Revised Arcane Value XP Costs:
In my own "Astonishing ACKS" campaign, I set up custom Arcane progressions which I've been happy with. Instead of increasing the power level of the lesser Arcane Values, you could calculate revised (lower) XP costs instead.
I still wanted to peg the full Arcane 4 progression at 2500XP (even though my house-ruled version is a bit more powerful). Using the "discounted spell value" approach, I found that my Arcane 3 Value was 51% as powerful as the "Full" progression, Arcane 2 was 30%, and Arcane 1 was 17%. This would suggest that the XP costs should be: 1276XP for Arcane 3, 754XP for Arcane 2, and 437XP for Arcane 1. But because I like round numbers I instead pick the following:
Arcane 4 - 2500XP
Arcane 3 - 1250XP
Arcane 2 - 750XP
Arcane 1 - 500XP
Compared to Axioms, this approach would yield partial casters with less spells but lower XP costs. The Warlock (Arcane 3), for example, would need 1450XP for 2nd level - compared to 2075XP from the ACKS Player's Companion. This feels about right for an impatient class who picks the "easy" path rather than seeking slow but eventual gains.
Revised Divine Value XP Costs:
My home campaign also uses custom Divine progressions. I've always thought that the default old-school Cleric was overpowered given his low XP cost, but I've been happy with Divine 4 casters needing 2000XP for 2nd level (compared with 2500XP for Mages). Let's peg my Divine 4 progression at 2000XP and see what the others should cost...
Using the discounted spell value calcs, I see that Divine 3 is 76% as powerful, Divine 2 is 51%, and Divine 1 is only 14%. This suggests XP costs of 1528, 1121, and 282 respectively. After some creative rounding, I decide on the following:
Divine 4 - 2000XP
Divine 3 - 1500XP
Divine 2 - 1000XP
Divine 4 - 250XP
That's a big jump between Divine 1 and Divine 2, but it feels right to me given the Cleric's power level. Using this approach, a Cleric would need 2000XP for 2nd level - the same as a Fighter.
I haven't gone through and updated all my custom classes yet, but when my PCs return to Hyperborea they'll find that some of the XP costs will be different.