Keys for maximum fermentability is making sure your themometers are accurate (as mentioned), mash longer, mash close to 148-150, make sure PH is in range of enzymes, make sure the mash is not way too thick ( this can prolong the conversion time which could in turn make your mash temp drop over time if you mash in a cooler) , and lastly you can use 6-row brewers malts or more "base" malt for an extra dose of enzymes. The problem may be you have a high percentage of unfermentable specialty malt.
As far as the iodine test goes, I say for the price of Iodine why not use it. I use the iodine that you buy in the drug store for like $2. I bought it 2 yrs ago and I am still using the same little bottle. I disagree that it is a useless test, it will tell you if there are major problems with your mash. It WILL tell you if there are any starches present in the wort. For instance if you have let the mash rest for n hour and you still are getting some purple tinge, it may be a sign that your themometers may be reading wrong or the PH is off and these factors may be inhibiting a full conversion. The downside, like Denny said, it will not tell you how the wort converted: into more dextrine or more fermentables. hope that helps some.