Averaged over 10 years / 70,000 miles of ownership, my 986S has cost me circa £1,200/year for maintenance, excluding tyres which I don't keep track of - fronts last at least 40,000 miles, rears about 17-18,000. I will say that I never bothered to fix the aircon when it failed as I can just open the roof, so there's a potential bill for several hundred pounds not included in that (the radiators at the front had rusted through and no longer hold the refrigerant). That's the water cooled flat six, 3.2L rather than the 3.4L or 3.6L in 996. I have had no major failures, typically just wearing items like the clutch. The previous owner asserted that neither the RMS or IMS had failed or been replaced.
I use an independent specialist and not an OPC, so labour rates are cheaper, but I don't get a bod in a suit come to tell me all the stuff the mechanic just told him about my car, or a leather sofa to sit on with a big flatscreen telly to watch while I wait for someone to talk to me.
Certainly not going to doubt your figures but those would be absolute best case scenario and as you stated, you've made compromises with selective maintenance, something I'd personally never advise on a Porsche. The Boxter, which even in S form, is somewhat cheaper to maintain than a 911. I think you've done very well out of that car though and obviously had no major failures. You must drive very very sensibly to get 40k miles out of a set of front tyres and 17k from rears...! I think the last rear set I went through lasted me just over 8k miles and I drive like an old man mostly. Spirited driving in the 911, more like 5k or less from a set of rears. I've seen people talking about 3k miles from a set of P Zero's on a 911 C4, at around £1,500 a set.... A good set of rears will cost in the region of £700+ so for average mileage, you might be looking at up to £2,000 a year in tyres alone if you drive a (non turbo) 911 C2 (double it for the C4!) with gusto.....
As an alternative comparison, a friend of mine has a 993 C2 Cab, which are known to be very reliable. Indeed, the air cooled 911 are fantastic. Despite this, his ownership costs for maintaining the car in proper condition are in the region of £6,000 a year, via a specialist. Granted it is a soft top and had a new roof, but even without that, it has not been cheap overall. He is now seeing a significant increase in the cars value though, as air cooled continue to become more and more desirable.
The main difference between a 911 and a Boxter in my opinion is that in general, when buying a 911, over any other Porsche model, either OPC or specific specialists (autofarm, portiacraft etc. ) are the only acceptable maintenance providers if you wish to retain resale values. Also, its important to make sure the car is as close to pristine as possible for the same reason. I like the Boxter, but it is not yet considered an icon like the 911 and buyers of 911's tend to be far more picky unless they are on a budget and I think people on a budget live to regret the purchase in most cases, as many a tale on pistonheads will tell.
I stick by what i said originally in that £5k a year is a decent estimate to run a 911, if you want to make sure it's fully functional and in good shape going forward, assuming no major disasters. If you come in under, its great, but to not budget for this would be folly.
For what it's worth, I would not own a Porsche (or any other serious performance/supercar) outside of warranty, under any circumstances. Best advice, go and luck at the price of parts, because whether sourced from OPC or independently, they will be roughly the same price and it would be utterly foolish to fit bargain basement after market parts to a 911, that is if you ever hope to sell it for anywhere near its potential value.
The reason I feel the above is relevant to the OP, aside from the fact he mentioned Porsche, is that an AMG Merc, RS Audi or M series BMW is likely to be the same in terms of maintenance cost, if not more.
I will go as far as to say that anyone who suggests you can or should run this type of car on a budget is living in cloud cuckoo land. Caveat emptor!
PS. Corrected typo and added some info.
(and I'll be quite jealous 
, even though I decided against an older 993 for the same sort of money when I bought the Boxster)
