I lost 1 Base Commander, 5 Generals (6 units in total) plus 7 other personas, so I understand the frustration!
Although there may be have been approx 250,000 registered users of the SGC, with people like me on there (with 13 personas!) it isn't a very representative figure of the actual number of users of the site. I would expect the actual number to be around half of that figure, and perhaps half to two thirds would be from the US and able to recieve Showtime (an educated guess). I would therefore expect the loss of subscribers to Showtime to be in the tens of thousands rather than hundreds.
You are correct nontheless in that Showtime will still have alienated a significant percentage of their customer base, and potentially lose those customers.