AFAIK nobody has tried this throughout the game, and then reported their results. However, I think the extra experience from killing strong monsters early is greater than your losses due to leveling faster and getting less experience from weaker monsters.
You get greater experience from quests at higher character levels, so that should also help a bit if you are leveling faster.

I don't think it makes a huge difference, though, either way. It can certainly help at the start of the game (see spoiler below), but by the end I'm not sure you would be significantly ahead.
The game gets easier as it progresses, anyway, so you don't really need to wring out every last experience point you can get.

Without leaving Aleroth, you can use the higher-level-opponent experience bonus to reach level 4 before you enter the catacombs.
Explore / loot around Aleroth and kill any rats, chickens or cows that you want (50 exp on level 1), but you may want to delay completing any quests. At level 1, if you defeat the zombie Jake (the graveyard would be easiest), you will get enough experience points to bring you up to level 3. This requires a lot of hit and run with a bow, or a moderate amount using Meteorstrike. Defeat him a second time (in his cellar) and you get to level 4.


The experience points required to reach a particular level are described in the topic:
Wisdom - is it worth the skill points?

The experience vs character/opponent level difference relationship for Beyond Divinity is described in the topic below. I did not check this for DD, but I'd expect it to be the same.
Act 2 - not getting exp in BF


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