Right so just caught up with the thread...first of all I'm with bigboss and I can't see JL matching that deal I'm afraid but then if you don't ask you don't get so it's well worth trying.
I think their response has been on one hand good and on the other bad in all honesty. The bad is the bit where they said you had already had your goodwill gesture, I'm of the opinion that if that original gesture of goodwill replacement then has issues then they should be more inclined to do all they can to help you. I find that stance a bit puzzling in all honesty as a faulty TV is a faulty TV full stop no?
The offer of renumeration and to let you hold on to the GT50 until the new year is a positive step forward though but as others have stated you need to get assurances in writing about that offer just to cover yourself in case they magically have no record of the offer when you take them up on it. I know it means more potential outlay but I really think they won't budge from that stance now and in all honesty you have done well to get that when the original issue goes back to a GT30 and I'm sure they are looking at your case as a whole here in making their decision.
It's a really difficult situation because JL could quite rightly tell you where to go if they wanted to as the faults are considered within spec although clearly there is something wrong with your set, a second opinion would be your only other option if you wanted to pursue it further and the outcome of that could potentially affect the offer you have already been given as they could withdraw it and even lower it depending on the result of the tests. Of course a second opinion could go in your favour but for me I don't think it's worth the time and hassle, after all the TV has to come off the wall again then go away for another 3 weeks or so and then you would have to negotiate all over again once the results were known.
In your situation I would take the offer and wait until gel gets the price down on the F8000 for you to a level where you can afford to splash the extra cash...I'm certain he will find a deal for you sooner rather than later and certainly before Christmas. It's not ideal I know but it is a result of sorts and the best you are going to get so take it, I would