In-memory OLTP is for very, very, very specific scenarios (very high speed inserts mostly). It's not a magic silver bullet for everything. It can easily be slower for inappropriate uses and it's got a lot of maintenance and monitoring requirements.

As for normal queries, you may see performance improvements just by moving to SQL 2014, you may not. Run the queries against 2008, run them against 2014 and see. Don't expect massive gains.