Any benefit to making a contributer lvl based giveaway

It's only a preference.

Well there's less likelihood of trouble, a.k.a. the winner not marking your gift as received or regifting it.

Less regifters/double winners and other undesirables at higher levels

You gradually narrow the amount of people who can have access to it, pretty much.

It's the matter of view. Should we really talk about benefits for the creator, well if you create a giveaway for higher levels, there is an advantage of taking less risk of dealing with potential trouble created either by a newbie unaware of the rules and/or unwilling to learn the rules or someone who simply ignores the rules purposely. As for benefits. Well are there any? That's a good question. The more you give away, the higher you go yourself. In general there are decent giveaways with less amount of entries on higher levels, so when you get there and manage to win such giveaway, for some this is a motivation to give back to the community and maybe create a giveaway for the same level group simply to make the other giveaway creators know that you care and appreciate what they are doing and furthermore, you can read on the forum that there are private giveaway groups that may invite you if they like what you do for others, so in general I would say it does matter what you do here and if you do something others might enjoy, it can be beneficial to you in the long run.

it encourages people to give away in order to level up, thus one might expect better ratios at higher levels

direct? none. Besides avoiding problems (your GA gets traded/regifted so someone earns CV/money on your cost, GA winner doesn't understand how the site works and doesn't mark your GA, or even never comes back to the site itself - you then have to bother with support tickets) - creating level restricted GA means that for one winner is not a newbie so he probabbly is aware of the site rules and two - he's probably frequent user meaning he will activate and mark the key/gift quite quick and without problems.

As for indirect thing - creating level restricted GAs motivate users to give away themselves so they can level up. The more GAs are there for more different levels the more whole userbase is motivated to level up, the morethey give, the more chances we all have to win. So it may not profit you alone but it does profit the whole community, including you as well :>