First off, ditch the K&N filter and restore the stock OEM CAI and filter as the oil from the K&N is known to degrade the MAF sensor to failure. A cone filter will actually rob you of power in the summer.
Secondly, the VH45DE has dual exhaust as it comes from the factory with very low restriction.
The Nissan catalytic convertors are very robust (why they are frequently stolen for their high platinum content) so seldom fail without abuse like running too long with a failed O2 sensor or non Tier 1 Premium fuel.
How about posting a maintenance history? Or got to Infiniti Onlin Mechanic Forum on
www.nicoclub.com and post the same information?