I actually think 10.0 will be a bit 'easier' than 9.99. The reason is the stock miner will have finally switched to searching for 10-chains in the sieve and it will find a few more 10 chains than when it was searching for 9-chains. Once the diff passes .96 I think it is better to search for 1-higher chains. The biggest factor is just the nature of division though.. going from 9.0 to 9.5 cuts in half the number of 9-chain-blocks you will find (from 100% to 50%). But so does going from 9.98 to 9.99 (from 2% to 1%)
I agree. My pool miner has already auto re-targeted to 10 (unless manually overridden), and I'm finding that the block rate is marginally better than if targeting the current network difficulty.. As I'm sure you're aware the difficulty calculation is not perfectly linear, so while the fractional part appears to indicate only 4% of 9ch's are valid now, it is actually less than 4% that is valid at the moment..
On the positive side, the step up to a target of 10.0 comes with a decent reduction in primes needed for sieving, as well each sieve returns fewer candidates for primality testing - so the raw number of candidates sieved/tested per second increases quite nicely.. This increased speed and the fact that *any* 10.0+ is currently a block solver makes it worth the switch IMO.. The way the pool miners work is that if a chain less than target but greater than submission difficulty is found the "share" is submitted.. In this instance, when solo mining with the pool miners the target can be set to 10, and while that sieves away a good percentage of the 9ch's.. it leaves enough for you to still encounter some block solving chains in the 9ch range..
Also, if you look at the difficulty charts from a historical perspective, once a target length is breeched the network difficulty quickly increases to the x.3-x.4 range - and I expect this to be the case with 10 as well..