There are a few performance comparisons between Dapper (which seems to be the fastest, most popular "micro ORM tool"). Now it's September 2014 and we have Entity Framework 6 (not 5, or 4) and Dapper is still around.
We will begin developing a huge database n-tier application (database has 700 tables). And some of the queries that need to be run are quite time-sensitive.
Ans: I don't have specific numbers, but I have updated the performance rig to EF6; I can't remember the outcome exactly, but: EF6 is a lot faster than EF-old, but dapper is still significantly faster in many scenarios (and about the same for the edge-cases)
Ans: That is subjective; for us: yes, absolutely worth it - but we care lots about speed. Parsing expression trees has impact, and generated SQL is very rarely in the same league as handcrafted SQL by a decent dev
Ans: Absolutely; I think I've deployed about 4 versions this month...
Ans: Yes, you can do that; that is how we started with dapper - we used that to replace LINQ to SQL code that was causing performance problems; over time, that had grown, and now we have very little LINQ to SQL code remaining (but still some)