However, that depends on the order you send LP messages.
If you order them higher hash rate first (like slush does) then those with a lower hash rate will get their LP that tiny bit later, if there are more higher hashing people on the pool. Of course this only means a small drop if it is the case.
Ok, true, more users in the pool affects your work reject ratio, but the difference will be tiny.
The BitMinter server orders long pollers by hashrate for getwork and getblocktemplate (GBT). For Stratum there is no ordering when pushing work (at least not yet).
Currently at each block change work is pushed to 7000-8000 Stratum connections in 0.01-0.03 seconds. 8000-9000 long pollers (getwork/GBT) are notified in 1.5-1.8 seconds. A few more ASICs running Stratum won't really make any difference.
Also of course without the source code one can't verify how the pool prioritises responses to miners.
But since you have to pay for the miner (it isn't free), I'd guess no one else has the source code

Yes, the server software is closed source. And it wouldn't make much difference if it was open source, because you would never know if the source code I gave you is what I was running on the server.
Not sure what you mean having to pay for the miner. There's no extra fee for using BitMinter client over other mining clients.