Not much in the A vs J, J are milder and more emission friendly as fitted to later 3.2. If your going to the trouble of swapping Cams I wouldnt swap the J,s for A,s, fit 4 x G,s, as you say your lose some lower end torque but more grunt at the top end. If your taking off or swapping Heads do a DIY port job on them.
I used to have a Ex Police 3.0 with 4 x Gs, and inlarged Trottle Bodies and a re Chip, went really well, plenty of Mid and Upper grunt, adding KN cold feed and Performance Exhaust improved it more.
My 3.2 Manual Ex Plod runs standard G and Js, also has Widened TBs and KN Cold Feed, ECU ReMap and Performace Exhaust, the ReMap and Exhaust made a big difference, and is still the quickist 3.2 Ive ever driven.