Am I correct in assuming that the Improved Familiar Bond feat would allow my Cleric 3/Wizard 3/Mystic Theurge 1 to count as a 7th-level character for determining the familiar's abilities? Does it matter at all that he already has levels in a class that grants a familiar?You can't take Improved Familiar Bond until you have Familiar Bond, and
Special: If you have (or later gain) levels in a class that grants a familiar, whenever you select a familiar, you can either base your familiar’s abilities on your total Hit Dice per this feat (including the restrictions on its special abilities), or choose to apply only your levels in classes that grant a familiar (and thus gain all the special abilities that familiar would grant based on those class levels). You can never have more than one familiar.
So (1) you got a familiar by being a wizard; (2) you take Familiar Bond and either shoot up to your full hd in 'wizard level' but lose most of the familiar's special abilities OR you stay right where you are in both; (3) you take Improved Familiar Bond and get to use your full hd and have all the familiar's special abilities.
For multiclass wizards Familiar Bond is basically a tax. Oh, and remember you need to take Iron Will to qualify for Familiar Bond.
P.S. The first option in (2) is not quite pointless since your familiar gets better int and natural armor as well as some of the special abilities.