If you want to min-max, you should optimally turn in quests as late as possible, because the XP from quests scales with character level. You can probably complete 1 and 2 stars right away, those don't give that much XP either way, but keeping 3+ stars for later turn ins might be viable for maximizing XP gain. And yes, as Blood Forest is pretty much exclusively 3+ star quests (the designer for that area didn't really bother assigning proper difficulty ratings to them unlike the rest of the game), it is best saved for last as far as XP goes.
As for the experience penalty, here is a graph:
from here:
https://www.sacredwiki.org/index.php/Sacred_2:Experience
As you can see, the penalty ramps up very quickly, with very noticeable jumps every 25 levels above 100.
As Hooyah said, adjusting the first number in MP_experience should be safe. The numbers say how much XP you get from kills, depending on the number of players in the game. 1000 is 100% for 1 player, 1150 is 115% for 2 players and so on. So by increasing the first number, you can easily tweak how much of an XP multiplier you want from kills. The XP penalty above level 100 will still apply though, so in later stages, even if you put like 4000 there, you will only be getting 40% of the kill XP at level 175.
As for ExpLS, I can't confirm that increasing them above 1000 is safe. It can very likely break some internal math as those are exponential multipliers and I honestly have not tested how numbers higher than 1000 behave. If you do try it, be absolutely sure to backup your save first as it might cause an overflow or something like that and cap your level instantly.
But simply removing the penalty does wonders for smooth levelling without requiring excessive amount of grinding. Up until the 160+ range or so. Then you have to grind either way.
You might also run into a problem where monsters in some areas will start hitting their level caps, effectively reducing farming effectiveness in that area. This is caused mostly by Survival Bonus, which can bump enemies 20-30+ levels above you once you are very high level, so that they start hitting area level caps. You can change the limits by raising the numbers for Spawn_OffsetHigh in balance.txt. It has 5 values, 1 for each difficulty respectively, so raising the last number will increase the max possible levels for monsters on Niobium. But this won't be a problem until the last stretch (level 180+ or so). Also, don't put that number too high, or monster levels will start overflowing back to 1, if the maximum area level surpasses 252.