That's why p2pmining or whatever exists. The problem is the 10 second LP. Slower miners wont finish work before its reset. Some of my slower miners ( a dual 5750 getting 300 mhash ) will only get 1 share on a good day at the default difficulty.
Except your math is nonsense.
300 MH/s is 2.6*10^13 hashes per 24 hours.
That is ~ 6000 diff 1 shares or ~ 7 diff 800 shares.
The odds of you getting 1 or less in 24 hours is 0.7%.
There is no such thing as progress towards a share. Each hash is independent it either is a share or it is worthless. You don't work towards a share so LP doesn't affect smaller miners any more than larger ones.
While it is true the variance for small miners on p2pool can be high your reasons are not.