I understanding what your saying and appreciate your expertise with them
However I hate to see a reefer pay 150 bucks and watch the anemone waste away.
If they take the food eagerly or have a response it is "one" way of telling if the specimen is healthy or not.
Beats seeing if it grabs onto your finger or not lol
Feeding a gig at the store is not a good idea.
Gigs are known to partially invert during shipment. I've had one completely open its mouth during a 30 minute drive home. Had it eaten it would've expelled the partially digested food into the bag.
Unfortunately, feeding a gig is not indicative of overall health. Unlike other anemones that will refuse food, gigantea will eat, but have been known to develop bacterial infections down the road -- which can take up to a month to show symptoms. I've had gigs that had voracious appetites yet still died even with antibiotic treatment. Feeding also requires the nem to use energy which it could otherwise use for acclimation purposes.
I actually prefer the "stickiness" method as it shows that the gig can still fire its nematocysts without it actually eating.
In terms of price, $150 is typically what I pay for gigs in my area. I've gotten a purple for as little as $100. Given the rarity, for $150 I would give it a shot, but then again I'm crazy like Orion, and I buy every gig I see regardless of condition (I used to think I could save all of them!).
To summarize, if you want a gig and the price is right for you, go for it but expect to treat it, and be happy if you don't have to. Just observe it while at the LFS and if it's acting normally, buy it. Its chances of survival are the same whether you feed it or not, and to me, feeding it has more risks than benefits.