Like the others, I would say it is worth it on the A340-500.

We've just returned from GLA-DXB-SYD in J class and it was excellent. On the way back on the SYD-DXB leg, my seat footrest broke about 7 hours into the 14 hour flight. The FA was very helpful and after deciding it couldn't be fixed, offered me the rest of the flight in F.

For a variety of reasons, including the look on my poor wife's face (there was only one F suite available) and the fact that my seat fixed itself half an hour later, I declined.

However, for that long segment, good as the business class seats are, I reckon the beds and pyjamas in First would have been excellent.

Aside from F vs J, I found the whole Emirates business class experience seamless and impressive:
Chauffeur drive from our home in Edinburgh to GLA
Lounge access at GLA
Priority boarding at GLA
Good J seat on the 777-300ER to DXB with full ICE with 500+ channels
Overnight in Dubai at Le Meridien courtesy of Emirates due to the 10hr layover
Re-entry to DXB in the morning through the dedicated First and Business class terminal
Excellent A340-500 J class 13.5 hr flight to SYD
Chauffeur drive at SYD to our hotel.

In any routing from GLA, the benefit in F will only really be from DXB to your final destination. As I'm sure you're aware, GLA only operates with a 2-class 777-300ER. From DXB, you also have to check the onward flights carefully. For example, there are 2 daily DXB-SYD flights. One with a 777W via BKK which has the newer First seats for a 777, but not the suites of the A340-500. Or there is an A340-500 flight direct to SYD complete with the sliding door suites. That plane's final destination is actually Christchurch.

Looking at Travelmood just now, I can see J clas flights to MEL from GLA for 2177 GBP and F to MEL at 2757 GBP. So if I'm right, that would make it 8k for four of you in business, or just over 11k for four of you in F.