I don't have reserved slot, but I play on the sourceop servers pretty regularly, and almost exclusively. So, along with Ret, i have a pretty unique perspective in the forums.
Basically I feel that if the reserved slots are increased to all 32, then it's not a donation--silver fox is right. It is a fee to play, and the server is not really a public server. New people are allowed on only if the server isn't full of donors, and can be kicked at any time. With the massive popularity of the servers at the moment, I feel that the actual probability of new people being able to play on the server is pretty slim.
But I also understand that if you donate, you expect to play on the server. That seems quite reasonable to me.
So I think increasing to 16 is the best move, since it allows more spots for donors, but doesn't make the whole server fee-based. However, this is no more than a stop-gap solution, since the real problem here is basic supply and demand. I know there is valvemaps #2, and i guess more should be added as required, with similar proportions of donors to non-donors.
just my 2 cents
oh and i'm gonna donate soon!