PP - having flown in and out of cloud in formations from 2 to 10 aircraft, including at the top of loops, probably around 98% of the time the cloud is 'thin' enough to see the target aircraft clearly. The problem becomes significant in large formations for the planes further removed from the leader and it is not unusual for some of these to be 'spat out' of the formation (and heated debriefs to take place
).
To sum up, generally not a problem, and NO - strobes not encouraged.