I have not convert any databases to ACE (.accdb/ 2007 format) and done a performance/benchmarking comparisons with JET 4.x.
In the few 2007 front ends that I have worked with that Access back end, I have not really noticed any differences in performance with queries against the tables in the back end using ACE (.ACCDB).
Note: I only work with split databases. The front end is an ACCDE (MDE) on the local workstation running the Access Runtime and the back end is on a Windows SBS (Server OS) in a share.