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To answer the OP's question I'd say the power output its comparable to 991.1 GT3 with using OE bottom parts for 4.0 build on a 997.1 GT3.
997.1 GT3 3.6 to 4.0 build using OE parts, stock 3.6 tune, stock 3.6 cams, TPC headers, TPC side muffler bypass, stock .2 center muffler. Could it be more powerful? Yes, of course. But due to racing series class rules that this car is intended for we have no need to go further. I know Dundon can easily surpass this with their proprietary-developed upgrade parts. Not trying to compete with other engine builders for power supremacy here. This is to demonstrate what can be achieved with OE bottom end engine part(4.0 crank, pistons, rods...same as factory 4.0 Cup cars that ran 24-hr races). Video clips below the dyno chart.
Apparently I missed the links for the dyno graphs in this thread. My apology. In the spirit of keeping things more consistent for the readers, I converted to CF:STD like the other dyno graphs.
997.1 GT3, 3.6 to 4.0 build, OE bottom end parts, stock cams, stock ECU tune, TPC headers, TPC side muffler bypass, .2 center muffler.
997.2 RS, 3.8 to 4.0 build, OE bottom end parts, stock cams, stock ECU tune, stock exhaust.
So yes, a 997 GT3 with 4.0 using minimum OE parts to build with stock ECU tune will produce comparable power to 991. More power can be had with custom tuning, more custom parts, and bigger displacement.