Two major problems, the first is in the effects of raising the dLVL cap, the second with opening the AoE can of worms.
With the dlvl cap at 10, everyone pretty much is guaranteed 25k an hour let's say. Doesn't matter if you stack archers, or manaburn, or tp burn, take 3 people, 5 people, 8 people...you're pretty much going to get what the mob spawn rate tells you you'll get. Sucks. But at least it's 25k.
Now, raise the dlvl cap to 20. Now rank 30's are fighting rank 50 mobs. Or at least they're trying to. This stresses job balance and effectiveness, especially when the focus is on kills per minute and sp per hour.
Not every job is good at fighting at a dLVL of 20 when the focus is max kills per hour. In fact, the stress of that change will cause more than one of the meleeing jobs to stall out and fail and in their place will be the jobs who shine at rapidly killing a dLVL of 20.
The jobs that get stressed at the dLVL of 20 will be dropped or left in town. Jobs that excell at dLVL of 20 sp'ing will stick together. You've created a Russian Feast or Famine economy where you're either fat or you're starving.
Old setup:
Mrd mrd Pug Rng Lnc con thm
Enter: stress of dlvl 20 partying
Conclusions: mrd pug lnc stink. arc shines. Con stinks. Thm shines. Need a real tank.
New setup: Gld pops and is instantly taken. Gld, knowing they're wanted, doesn't party with setups that aren't heavy in arcs and thms. Anyone who'se not an arc or thm has harder time landing gld players. Thm, gld, and arc stick together to maximize their sp per our in this new stress level, leaving mrd pug lnc and con to fend for themselves.
Old setup: everyone gets 25k an hour
New setup: two kinds of parties. Ones that get 45k an hour and ones that get 15k an hour.
This was a real phenomenon in FFXI. Rangers got ranger ranger ranger brd brd cor parties. Dragoons got whm whm pld thf drg blu parties because that is the only thing that's left after stress divides the population.
And the second...you can't unleash mages to indiscriminately AoE the SP world. Summoner burns are already raring to go, and it was already abused to hell in XI with xp-getting, farming...everything.