Who knows? Maby adding the V-8 might actually improve the 911's F/R weight ratio.
Porsche could choose to follow an example set by the Domestic car company Chevrolet

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Chevrolet has cut the weight of the smallblock castiron engine by a whopping 100 Lbs by replacing the castiron block with aluminum bringing the weight down from 490 Lbs for a fully dressed engine with heads and intake to 390 Lbs.
If Im correct, the standard non turbo 911 engine weighs around 410 Lbs add a turbo or two and the weight will jump significantly.
Now take the new Chevrolet engine, add lightweight pistons & rods, shave the crankshaft, add magnesium heads & carbonfiber intake, then have the engine internally ballanced, just like F-1 cars, so as to eliminate the harmonic balancer. With these additions, or should I say subtractions, the weight should drop another 40 to 50 Lbs bringing the weight to around 350 Lbs.
And what does that last paragraph have to do with Porsche???? Everything, Porsche could do to their new V-8 what Chevrolet did for theirs plus a whole lot more.