Ive seen alot of the trickier players create a little space so the racket/hands are easier to manoeuvre, but as for viability... its really up to the player... you need to be really relaxed to play with such a relaxed hand grip. Imo Not advised.
IMHO if u want full maximum power on yr overhead shot (ie smashes) u may want to tighten a bit than that (having a bit of space in between yr palm n the grip). It all depends on yr stroke techniques n yr power. The pros can be more relaxed in doing overhead shots because they have plenty of power and good stroke techniques. Most importantly is that your racket face have to get BEHIND the shuttles and have enough swing speed to hit good overhead shots.