A few guesses (based on very limited information provided and a few answered questions):

1) You have much faster IO now and it is able to "swap out data" in RAM (i.e. read it off disk) much more quickly. This can lead to lower PLE although query performance would still likely be better.

2) Another possibility is that someone messed with the schema and dropped one or more important indexes from large tables, leading to scans instead of seeks.

3) Oh, what about MAXDOP server setting?? If you had it set to 1 (not uncommon in the ERP world) and now it is back to default of 0, you would be parallelizing potentially many more queries, which would be consuming data more quickly.

4) Yet another is that you simply did monitoring while some very large reports were being run for some muckety-muck VP. 🙂

5) It used to be that upgrading editions meant having to do a full-scan statistics update on all indexes/stats. But I don't think that is required when going from SQL 2008R2 to 2012.