Not saying I hold the truth... It's just that I am very careful with using flank speed when submerged because it seems to me from various experiences (usually bad ones) that it kind of sends a signal around "hey, I'm here !"
If it's a lone escort and you are sure to be in his baffles, it should work, but if there are several escorts... You can hear a ship prop 15 / 20 kms away in good conditions, there is no reason why the hydrophone operators on the other side should be deaf...
In some conditions (night and light fog for instance), if you had to shoot from long distance for positioning reasons, I think it is even safer to get away surfaced at slow speed (because of the wake) than to submerge at all...
PS : and if the escort are only Flowers, even if they start turning your way, you can also outrun them as you are faster surfaced at flank speed.