I have a few house rules to deal with spell scaling and armor issues, all based around the idea that the more experienced you become, the less the limitations affect you.
First, I have two skills called "Armored Casting" and "Spell Scaling". Both Mystical Arts SD+In. They work like a hybrid of the Blindfighting and Ambush or Sniping skills in that they a) reduce a penalty but b) their effect is based on the number of skill ranks rather than the total skill bonus.
Armored Casting reduces the casting penalty for wearing armor by 1PP for every 7.5 ranks (round down) in the skill. -1 at 7 ranks, -2 at 15, -3 at 22, -4 at 30 ranks, etc. This can, at most, reduce a casting penalty to 0, it can never provide a bonus. Why 7.5 rather than 5 or 10? 5 is far too cheap, 10 is a little too expensive.
Paladins from Harpers Bazaar can also develop this skill and it stacks with their casting penalty reduction - e.g. at 4th level, they automatically reduce the CP by 1 just for being a Paladin AND they can have a maximum of 15 ranks in this skill, reducing the CP by another 2. (a minor house rule I have is that the Paladin ability can also reduce the CP to a minimum of 0 rather than a minimum of 1).
Spell Scaling reduces the casting penalty for scaling any spell by 5 for every 5 ranks. This only reduces casting penalties from scaling spells, not from any other source and, at best, can reduce the penalty to 0. It can never provide a casting bonus.
I experimented with requiring a maneuver roll to activate these but found that it wasn't worth the bother, it just slowed down the game. Anyone with a low skill bonus is only going to get a minor benefit (enough to, e.g., reduce the casting penalty for soft leather to 1 or 0 PP, which they can mostly achieve with armor by the piece anyway), and those with high skills are almost always going to succeed.
I also tried Spell Scaling as reducing the scaling penalty by 1 per rank, but I preferred the punctuated equilibrium effect of 5 per 5 ranks. Gives players something to aim for.
Second, I have a house rule that the better you are at a spell, the faster you can cast it. Practice makes perfect. For every 5 full ranks you have in a spell, you can cast up to an extra 1 PP worth of that spell (and ONLY that spell) per round. So if you have 6 ranks in the Light spell (the base PP) you can cast it in 1 round, and 6 ranks of, e.g., Elemental Bolt (Fire) lets you cast it scaled up 1 size level (base 4 PP, +2 PP scaling) in 1 round. If you have 10 ranks in Major Healing or whatever, you can cast up to 7 PP of that spell incl. scaling in 1 round. 8, 9, or 10 PP worth of scaling would still take 2 rounds. If you have 15 ranks in a spell, you can cast up to 8 PP of that spell per round, 9-15 PP takes 2 rounds (because at 8 PP/round you can cast up to 16 PP in two rounds).
BTW, I don't see this as "unbalancing" because IMO balance in HARP comes from the fact that everything you choose to improve comes with the opportunity cost of not improving other things. If a caster wants to spend 30 DP over four or more levels to get 15 ranks of Armored Casting in order to reduce their armor Casting Penalty by 2 so they can wear soft leather without penalty, that's fine. That's 30 DP worth of spells (e.g. 15 ranks of Mage Armor or Tree Skin) they haven't developed. That's 3/4 of the Eloquence talent. That's an additional Sphere they could have learned instead. Similarly, if they want to be better at scaling spells in general, it costs DP that they could otherwise spend on gaining more spells or improving the ones they have.
Note: I am using the old HARP rules and supplements from a decade ago. One of these days (after the upcoming Bestiary and Something Wicked are published) I'll probably get the new versions but there doesn't seem to be any compelling reason to do so. And from what I've read, there are no major changes in the current version that would be incompatible with the above house rules.