I read through several threads here for IR information (there's several very good threads with lots of information). Here's the one I found with the list of IR friendly and unfriendly lenses.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/66895-ir-infra...ax-lenses.html .
Mike - I know what you mean about smaller lens hoods, found it out the first time I tried to put the lens hood on the DA*50-135 (67 mm) when I had a 77 mm circular polarizer on it (lens caps are a problem too - I really need to buy a generic 77 mm lens hood for the filter).
The more I think about it, the more I think it makes sense just to get the 55 and the extra step-up rings. Getting a 77 mm R72 filter is probably around half the cost of getting a camera modified and I have no idea if I'll like doing IR enough to spend that kind of money. Glad to know that step-up rings won't be a problem.