hey im also pretty new to the thundercat, i have had mine for bout 1 month now. i was looking at two at the time one on a r'reg with 30k and one on a p'reg with 16k. the r'reg had a few things extra which i wonted but was attracted to the p'reg just cause of the mileage, it has its service history so its not clocked.
just had service - oil, filters, spark plugs, carbs cleaned and balanced, new chain and sprockets.
i was worried bout the box but the same as you i give it some in second on the test ride and it didnt pop out so thats good its just a little harder getting it in second than other gears but only just. after a week or two started riding a lil funny i thought to box was on the way out but it was just the chain, it was knackered. all good now.
role on summer!