I've not used the elastics yet, though they look good quality and sensibly laid out.
What I have done is shorten the pannier lid strap by about 100mm.
As someone here has discovered, if the lid drops, it'll travel so far it'll rip the rivet out and let the lid hit the tarmac.
I have pannier liners, so I keep the panniers empty most of the time. I adjusted it just to get them/helmet in.
Triumph have thoughtfully put a buckle in the strap, so if you need to open it wide you can effectively remove the strap altogether. Nice feature.
Being able to get a helmet in the panniers was a major consideration for me.
A pair of size 11 Alpinestars and (armoured) leather trousers fit easily in one pannier with space to spare.
A flip-face lid and gloves easily fit in the other. Not quite enough room for a jacket as well, but if you used the helmet hook you'd get all your bike gear stowed. With a top-box, me and Mrs TB can leave all our kit with the bike.
If you need to carry a laptop bigger than about 12" in a pannier you may have a problem.
The top box looks fantastic, but we found on the SE test rides you can't fit two FF helmets in it, at least not ours.
There is a lot of lost volume because of the 'lining'. I'll get one when funds allow because they do look good and it offers better support for the pillion, but don't expect stuff to disappear in there....