If you lost enough torque to feel a "drastic difference", you're talking about a drop of like 10lb/ft+, not 1-2. There's simply no way the exhaust caused you to lose that much torque.
It's either perception from the extra noise, or the ECU needing some more drive cycles to adapt to the new parts. Give it a few more drives to see if it improves. If you REALLY feel like you've lost a noticeable amount of power then have it dynoed back to back with the stock vs perrin exhaust and see what the dyno says, it won't lie to you.
Here's Perrin's published dyno chart:
