Hi Rob,
I don't have any empirical data specific to the 8.13:1 engine in 3.5 litre format. I do however have information that is specific to the 4.6 litre engine in both low compression 8.13:1 and high comp format. Whilst it is true that the higher compression engine will deliver a greater maximum power at maximum rpm, it is the torque delivery that is most important for a road car. The low comp engine delivers torque at a greater rate up to peak torque compared to the high comp engine. This means that if your car runs an automatic transmission, it will be more pleasurable to drive as you will require less throttle to achieve the same degree of acceleration. The high comp engines are preferrable if your car runs a manual transmission.
If you decide that you do want to increase the compression ratio, then assuming you don't change the type of head gasket, then changing the pistons is all you need to do to either increase or decrease the compression. It is true that to make the engine tractable, you will need to change the carburettor settings such as needles, and the distributor advance curve will ideally also require changing.
Ron.