How long do you give a new ski when demoing before writing it off ?

I will guarantee you one thing, you are faster on your V7 upright than your EVO upside-down. Who advised you on such a huge step up on ski performance? There are many skis that fall in before that one for stability. For instance the V8 will get you to over 14km/hr in a downwind.

I have stated before your demo paddle should reflect on how much you paddle, if you paddle out and back for 2min then that is all you should do, however I generally paddle a bit longer than that and get tired so my advise is to get tired and then see how good the ski feels.

You also have to work out if falling in is a part of ski paddling, generally it is not but each one to their own. Also is it fun falling in?, I paddle for the enjoyment and find NO enjoyment in swimming.

Ranga
I don't fall off the Evo and I am not slower upwind in it than in a V7.
In the flat I am able to take selfies and check work emails. I felt and occasional wobble first few sessions, but its improving each time I am out.
I am good and thanks for your concern. It's an appropriate challenge and V8 would have been going in circles.

Sorry I thought you said you were falling out. 
What I meant was if you were falling out, swimming is not faster than the V7, no matter what ski.

No problem, I wasn't very clear.
I am beginning to enjoy the boat quite a bit.

The latest on my own situation with this is that after 2 months with the Nelo 550, I decided to be cautious and opted to buy the Epic V10 Sport instead. As much as I love the Nelo's paddling position and build quality, I can handle the V10 Sport in my local waters but the Nelo was still too much after around 15 hours/8 weeks of time in it.

This has raised another dilemma now. I think I'd like to buy a second boat intended for mainly flatter conditions and for 'getting the miles in' over winter. It will also be a 'challenge' boat that will serve to be used for balance training rather than racing and be something to get better at over a much longer period of time while I race the V10 Sport. I have two options. One is to buy the Nelo 550L anyway as a second boat (not cheap but I do like it). Secondly, I can get hold of a Epic V10 G2 in a black tip relatively cheaply. I'm not sure though which one to get and I haven't paddled a V10 G2 before (I have paddled a G3)......... any thoughts ? A V10 is much heavier and a pain to lift around than the Nelo. However, I'm 90kg and worried that I won't get the best out of the Nelo as am I too big for a ski that small ? - when eventually racing on flatter waters (The Nelo was designed for downwinding really but that won't be how I'd use it).

I was at a race this past weekend and the top three paddlers were in Epic v10's. I came in 4th in my Epic v8pro. The race was in a tidal river with small  boat traffic and about 1' chop. The top finisher told me he was looking for an Epic v8pro for paddling in rougher conditions. I told him just the opposite. I love my v8pro, but if money were no object I'd have a v10 just for what I'd consider flatwater racing. I do OK in the v10 up to about 1' chop. After that I'm a little twitchy. 

I'm sure others will tell you it's just time in the seat that will make you a better paddler, but if you can afford two boats -go for it. I feel confident to paddle my v8pro year round in the cold NorthEast US. I don't think I'd trust myself in anything more advanced in the winter.