I will have had my black SRT6 coupe for a year next month. I tried a couple of standard cars first which were a nice drive but I realised there was not much improvement on my twin-turbo diesel Volvo, so I know what you mean about not being that quick.
I've only done about 2k so far, so I'm only just getting used to what it's capable of (which is a lot!) and not had it serviced yet so can't comment on that - it's due in abut 1500 miles according to the FSS and it's over year since the last one - so I'll get that done before taking it off the road for the winter - tax expires end Sept. which should work out nicely.
So far no issues (fingers crossed - I have to remind myself that for every problem posted on the forum there are many more owners enjoying trouble-free Crossfire motoring).
It's a seriously fast and deeply satisfying experience - although I can understand those standard Crossfire owners who have 'upgraded' to an SRT who find it a bit of a culture shock - 'cos it is pretty hard-core by comparison!
So if it's a lot more power you're after, go for it - I've had no regrets so far - but obviously you need to drive one to be sure.
A few months ago, there were more SRT6 convertibles on ebay & Autotrader than coupes but lately they seem to have dried up - which is not surprising when you consider that only about 117 rhd SRT6s were imported to the UK - approx 75 coupes and around 40 'verts - rare cars indeed - no wonder you're having trouble finding one!
But keep looking - they do come up - the guy I bought mine from actually had two for sale...