I had this same 'dilemma' last month. My Turbo needed it's 4 yearly service, which includes a plug change and as we all know this is very time consuming.
As my car has all OPC stamps and as it is after all classed as a Supercar, I thought it was pretty much a no brainer that keeping the Porsche stamp was more important than saving £400 or thereabouts.
When I bought the car the first thing I asked was does it have a full Porsche history and it did, so I bought it.
Now please don't take this the wrong way and I know some will but when I see people cutting corners on servicing and maintenance a little alarm bell starts ringing and I ask myself what else have they scrimped on to save money.
You also have to factor in the car you have and it's value in the market place. Obviously a 97,000 mile C2 that's 12 years old isn't going to lose much, if anything in value by going to a reputable indy but a high end car is.
My only criticism of the OPC service is that during my cars visit I received a phone call form the lady in the service dept. She had quite a serious tone and told me of a number of problems that had been found and must be fixed.
1. A nail in my rear tyre, no problem I said just get it repaired. Oh we don't repair we fit a new tyre, oh well in that case leave it. Turns out it was actually a small piece of metal that had got pushed into the tread, I removed it and it had only gone in 2mm so no damage to the carcass and no flat. Saved £300.
2. A leak with misting on the turbo pipe and if we remove the pipe it will break as it has a crack in it. Which pipe I asked, the low pressure side or the high pressure side, this stumped her so she handed me over to the mechanic. Actual problem was the low pressure pipe that comes down to the inlet of the turbo unit, it had a crack in the fixing lug not in the actual pipe. So I asked how can it be leaking out ward as stated if it's the low pressure side, surely it would draw air in not out! Mechanic agreed to change the O ring, for free! and see if it was okay, it was. Saved £350 + vat
3. Condenser leak on AC, well not so much an actual hissing leak but what looked like the start of a leak, So I asked what's the pressure showing on the diagnostic, turns out it was at the factory pressure which I 'think' is nearly 6 bar. So no leak then? Err well it's just to let you know that it may go in the future. Saved £650 + vat.
Now if I wasn't mechanically minded and hadn't had my car to bits on a number of occasions I would have probably said go ahead and fix those items, so I would have had my pants taken down for an extra £1000.
I know they are at liberty to 'warn' you of problems but they do tend to come on strong with the 'urgent must be fixed' phone call.