Mirroring is performing the same. The memory upgrade has done wonders for performance actually. The CPU spike is the mystery --- I'm using Spotlight for SQL Server, and just watching CPU hover at 90+. The process using the CPU is this BRKR TASK, with CPU at 5,950,000, logical reads at 206 (has never changed) and zero reads and zero writes. The Last Wait Type is BROKER_TASK_STOP.

Aside from that, nothing else has changed. All of our reporting/queries are done on the mirror with snapshots so nothing but the application touches production.