Well, thanks to all for the feedback. I have pulled the trigger on a purchase and this is my overview of the journey...
Important note: To qualify my thoughts and findings, my cycling history has been limited to riding two carbon road bikes - a 2010 Merida Ultegra R1 (5000kms) and a 2011 Giant TCR Advanced 1 (28,000kms). I'm sure this info will assist some readers in the same boat as me, and maybe not others. Worth noting... I will say that I know my Giant TCR intimately! I 'Everested' 278km at 9300 vM, and completed an Ironman on the TCR (35kph ave for 180kms - sub 10 hr Ironman) on this bike so we are pretty tight about now...
Mission - to find a comfortable performance bike that could deliver in style and performance - hills/flats (refer to OP).
Test Rides:
I took a 2015 Cervelo R5 (Dura Ace mech) and a 2014 Cervelo S5 (Ultegra mech) out for a serious test ride on the same circuit. The ride involved flats, climbs (peaked at 15%) and importantly for me, crappy road surfaces (!) because that's the world we live in.
Gear consistency: Tyre PSI 100 for both machines. Both bikes had Mavic tyres and wheels (which I don't rate, however, as a control my current TCR has Mavic too).
The first ride of the two was the R5. The R5 had the feistiness/twitchiness of the TCR but was 'moderately' smoother on the ride. I was expecting more comfort/smoothness from this machine based on some reviews, but other online feedback had me thinking my experience was about right in an un-romanced 'real world' situation. My rationale - If you want total power conversion at sub 7kgs on a steep climb, it'll be a stiff ride and will cost you in comfort. Post ride, I did find myself thinking that TCR Advanced owners are getting a lot of R5 for half the price. In saying that, the R5 was was unquestionably superior to my uneducated senses... was it worth $7k plus though? What was better? Ride quality (all terrain) and response under pressure up a hill proved the R5 superior but I was not going to throw down $7.5k for it.
The second ride was the Cervelo S5. It was noticeably more comfortable/smoother than I expected, and more so than my current 2011 Giant TCR Advanced 1. I was well pleased and surprised by this finding, and I found it smoother than the R5. *Both bikes were on mid range Mavic wheels (I have Ksyrium Elite SL on the TCR). I had an instant connection with this bike which was a total shock. Admittedly with the S5 being an aero bike, I went into this ride expecting to be slightly disappointed by the feel, geometry, snappiness and climbing potential off the back of the R5 but I was proven wrong. The R5 opened up at 15% where the S5 became a touch cumbersome, but the S5 was certainly a climber despite weighing in at more than my 4 year old TCR frame.
My purchase - a 2014 Cervelo S5 mechanical Ultegra with Mavic Cosmic wheels (all parts/bike new). The purchase price gave me ample room to move with quality wheels in the near future to conform with my budget (detailed earlier - Dura Ace C35s etc).
After the honeymoon - sun, rain, hills, flats, solo, group:
I love the S5. It's a stunning bike in both ride and design. I'm not a pro but I have a lot of fun on undulating group rides. Last week I got a feel for where this bike is at on two very different rides... in one situation, I felt the challenges of the S5 over a sustained 12% climb and asked myself some tough questions about whether I had made the right decision, and on another ride (an undulating group ride), I 'won' the final 4.5% 1km climbing sprint at an ave speed of 34kph after 2 hours of rolling hills. I seriously doubt that I would have been able to do this previously on my TCR. The acceleration of this bike out of a bunch on a 5% gradient was awesome. This was a satisfying moment!
Ultimately, for me it's about getting out there and being in the moment and i'm really happy with my choice!
*speeds and gradients are only provided so readers can be informed on as to what wattage and gradients I realised my gains. I know these efforts are sedentry for some... that's ok!