If you are using SMF 1.1.x I am not sure how well it will work. This mod adds features to the existing newsletter system of SMF.
In SMF 2.0 there is mail queue system to send out email which might help with the large amount of members.
We use cronjobs only for sending scheduled mailings that are scheduled ahead of times.
Yes i'm using 1.1.11 i'm not ready to use SMF 2 untill a snable version is realese.