... big, old, inefficient engine and pointless 4WD drivetrain. In came a race-bred, smaller, more efficient engine and proper RWD so all that power was channelled correctly. Add to that fantastic styling. End result? Fastest car in the world. Made by a bunch of blokes in their spare time on a Saturday.
More than a touch of the rose-tinted glasses here methinks, an alternative view of the engine is: "Austin Rover withdrew from the rallying scene at the end of the season, but in 1987 all the parts and engines were sold to Tom Walkinshaw Racing, whereupon the V6 engine reappeared under the bonnet of the Jaguar XJ220, this time with turbochargers added."
And, sorry to be a pedant, but the XJR-15 was not a 'real' Jaguar, it was TWR (and sold more as a racing car with its own marque series). Notorious for its pendulum-like handling, which is partially a characteristic of the V12 engine. The XJ220 doesn't have that problem.
Sounds like your TWR is better than the other TWR!

From what I remember at the time, Jaguar had to threaten many 'customers' with legal action to get them to take the cars that were ordered. This led to disgruntled customers, many of whom launched court cases against Jaguar for the radical changes in spec, only to lose, and this led ultimately to unsold 220s. To reduce costs the use of parts from mass production cars had been extensive; for example the rear view mirrors came from the Citroën CX 2 Series. It never was capable of the targeted 220 mph top speed either.
Compared to sports Jaguars from the past the car can only be judged as a failure even though a total of 281 cars were made and by 1997, few of these remained available for sale new at £150,000 (launch price £361,000). Nowadays, it remains a somewhat sought-after collectible sports car, fetching £100,000+, but really many feel that this is more due to its idiosyncrasies rather than its out and out merits.