I don't think it's all that bad an idea to lower the spell levels. That being said, I've put a lot of work into estimating appropriate spells per level, so what I'd rather do is create a LESSER fireball, 1st level, 10ft radius burst and medium range, with cap at 5 dice. Lightning Bolt could be dropped to 1, since line effects aren't all that great.
For 1 die per level effects, I calculated that 1-3 level spells can be d6, while 4-6 should be d8 and 7-9 should be d10s.
This came from my assumption that each spell level is 50% better than the previous, for the levels they are in use, at least. Adding to this the level scale when spells of that level are the strongest spells...
At 2nd level, you're dealing 2d6 damage, typically. 2-12 range. Thus, level 2 spells should be at 3-18 range on levels 3-4, and...
lv 5-6, 3rd: 4-24
lv 7-8 4th: 6-36
lv 9-10 5th: 8-48
lv 11-12 6th: 12-72
lv 13-14 7th: 16-96
lv 15-16 8th: 24-144
lv 17-18 9th: 32-192
lv 19-20 10th: 48-288
An average CR 17-18 monster has a bit less than 300 HP. Average damage result of a spell of that level is 112, meaning a mage can solo an appropriate-level encounter in 2-3 rounds, using direct damage. At least that's what's _appropriate_.
Another patterns I've recognized are that mass versions of spells seem to be +2 levels in newer books, making mass cures underpowered by default. Also, Lesser/greater versions of spells are +/- 2 levels, and... well, vary wildly, but in mage armor's case, is +50% more powerful in greater version.