Is there only so much you can do with 6 cylinders?
 

Blancpain GT Sprint series. Not one Porsche. All other major players are there.

Is the 6 falling behind the HP / modern racing game?

Look at the Porsche Cup and Supercup cars. Porsche cars race in a lot of other series.

There in a lot of other series. Continental sports car, Tudor IMSA GTLM, and Prototype, Pirelli World Challenge.

However, they seem less competitive, it's time Porsche bring back the flat 8.

I think it's more a case of the rear engined platform having trouble against mid engine. Most race series restrict the air inlets to the motor anyway to enforce a balance of performance.

I've watched this series several times and I have seen two GT3's a few times of which Nick Tandy was one of the drivers. They have not been competitive in this series.

A rear engine Porsche will never win the overall at Le Mans or reclaim the N-ring lap record. Even if they could build a flat 6 991 GT3 engine that didn't catch fire, the mid-engine competition are simply too far ahead. Seems like the there's only so much you can do with a rear engine platform.
Put it this way if Porsche Motorsports had to start from scratch tomorrow with the primary goal of winning every sprint and endurance race the Boxster/Coxster would be the car.

IMHO sport car racing competitiveness is much more complicated than simply mid-engine vs rear engine.

Modern day rules packages constrain the car configuration in numerous ways such as power/weight ratio, tires, aero/downforce, energy use/energy recovery, etc. The extent of the rules are amazingly broad and often quite detailed. The days of simply racing whatever is "fastest" are long, long gone.

Thus, the winning teams are typically those that have the most money to take the best overall advantage of the rules package.

Exactly ! This is not heads-up, off the showroom floor racing, or a free design exercise.

Well I would have to say yes if one is referencing the available maximum displacements (3.8 ...maybe four) .

I am going by what hear from a Cayman guy in the World Chall. With a 3.4 one just can't keep up to the Mustangs and Camaros