Sport Auto tests the AMV8-4.7
 

Thought I'd share the info. I've put in for comparison, some of their other tests.
..............AM4.7.... AM4.3.... DB9.... R8
0-100k.... 5.2......... 5.5....... 5.3..... 4.7
0-160k... 11.3....... 11.8...... 11.2... 10.5
0-200k... 17.6....... 18.8...... 16.5... 16.2

Windingroad.com says

"The updates result in a fine blend of ride and handling, while the muscular 4.7-liter V-8 powerplant is simply storming. It might still not have the measure of a 911 Carrera S dynamically, but it has just taken a very big step closer."

Impressive numbers! It is incredible how much top end the DB9 has. It is funny what an extra couple cylinders can do for that. ;)

Seems like the vantage keeps up with the DB9 very well up to 160kmh...
It should! it weighs much less. They've really done some great tweaks with the 09 4.7 facelift!

I do not have the numbers but would guess my RSC modded 4.3 is something like a 4.7 ... or possibly a bit better ... :)

Prodrive says this about their 4.3 upgrade

With the performance pack fitted engine output increases to 425 bhp (317 kW), while maximum torque rises to 440 Nm (325 lbft)

On the road these improvements reduce the 0-60 mph (97 kph) time by 0.4 secs, the 0-100 mph (161 kph) by 0.9 secs and the 0-130 mph (209 kph) by 2.4 secs*.

... but warranty problems!

I know that a lot of dealerships for Subaru will actually back prodrive parts in the UK. I am not sure about how picky they are about the AMs, but Prodrive has been a legitimate race company for quite some time. You might get lucky and be able to talk your dealer into helping you out.

The prodrive upgrades are VERY uncommon. You will also find that when you decide to sell it on, there won't be many buyers for such a heavily modified vantage when there are plenty of clean, unmodded ones.

Save yourself time and money and go for an 09 (420hp/sportspack), now or when prices come down.