So you are saying it is pure luck factor. More TH/s you throw at it, more chances you have?
In other words, you could get solo block with the rig at 5GH/s, if you are extreeeeeeeemely lucky.
If you're throwing 5ghs at the pool you WILL eventually hit a block. Heck, I hit with a simple 400 ghs S3. The thing is it may take you 5-50 years to hit or it may take you 30 days. It's all a numbers game.
The analogy I like to use is scratch-off lottery tickets. If you sit at the counter scratching tickets and you can do 10 per hour your chance of winning is pretty slim. If you clone yourself 100 times and 100 of you sit there scratching tickets your chances are much more improved. You can win much more often.
More TH/s = more clones of you. If you only have 400 gh/s then fire them up! Just don't count on a quick win. If you have 10 TH/s fire it up too! And your chances are much higher that you'll pull a winning ticket.