Unless you've pumped up the power or made it a stroker, the header will do very little over the stock exhaust other than drain your wallet. Borla was the OEM manufacturer of many years of XJs.
I do mean the power steering fluid which goes into the box. They switched from an ATF thickness oil to equivalent of 30W which is 3 times as thick. Switching it to Amsoil PS fluid makes many rigs easier to turn
They switched to thicker oil. I'd change fluid and lift it off the ground to see if it still feels heavy. Make sure the tierods and balljoints are all greased.