Back to OP question....
I believe cars with OEM spec will be more sought after in years to come (how many years is still a debate) - However.....
Concourse competitions have usually two classes - Original & Modified - and very good 916s will start appearing in concourse events, if they haven't already, so there will always be interest in modified cars
When it comes to mods that cannot be 'readily' seen - Q2/ Quaife diffs, suspension bushes, chipping, uprated brakes etc - then its probably down to the individual taste and some mods will always be accepted because they 'enhance' the car - but don't throw away that broken oem radio - it can be repaired and can be refitted at a later date
I'm in a dilema... I have a Lightning Blue V6 Spider with 35k miles and is in 'pretty good' condition and I'm planning to keep this car and will bring her up to 'very good' condition with some tidying:
Engine bay - new oem inlet chrome pipes, cleaned up rocker covers, new air pipes and a good old clean
Interior - new kick covers, replacement door card and other bits an bobs
Exterior - repainted bonnet & bumper (stone chips) & other small dents around the car
This is on top of the usual maintenance tasks - suspension bushes, brakes etc
I also have a complete beige interior from my 'spares' car with a brand new carpet, which I was planning to get the seats refurbished and then fit into the road going car
Should I change the colour of the interior to beige? Blue V6 Spiders were supplied with beige interiors - BUT with a blue roof - I guess I can always get an after-market new blue roof (assuming finances allow)? But will it be an original car?