No - having seen both cars in the flesh (admittedly not together) I can assure you they are quite dissimilar.
The glasshouses of the two cars are similar - I will grant you that.
The Jaguar is much smaller than the R390 and the cabin sits much further forward. Also, the rear deck of the XJR15 is quite flat from the back of the glasshouse (no mean feat considering there's a friggin' great V12 in there).
I saw the XJR15 run at Albert Park in '93 and it is a gorgeous car in the flesh - I know there was some talk at the time that it was probably a more representative Jaguar supercar than the XJ220.
Peter Stevens designed the XJR15, as well as the front drive Lotus Elan and the McLaren F1. Tony Southgate, the designer of the R390 also designed the Aston Martin DB7 as well as a number of F1 cars from the 70's and 80's.
Coming from similar British backgrounds, I imagine there would be some similar elements in there.
