which engine is a better preformer which ca be upgraded further?

I'm not sure. Jaguar stopped making their V-12 because the V-12 would not have been able to pass new emissions and what not. Here is some information on Jaguar's V-12.

http://www.jagweb.com/jagworld/v12-engine/index.html

http://www.jagweb.com/aj6eng/stretching.html

The Jaguar XJR-9 (race car) had 750 hp.

The Jaguar XJR-9 (race car) had 750 hp.
I've owned Jags and BMWs - both great marques. That said, the 4 cylinder BMW M10 block (2002 and 320i) produced 1300 hp with turbocharging in Formula I qualifying trim back in the 70's.