Ben, does your testing confirm the below situation?
Increasing the character length will involve making changes to the StringValue database table. The catch in this is that all the string valued attributes across Windchill are stored in this table. Increasing the character limit on this table to 4k, would mean we are increasing the character limit for all the string attributes in Windchill to 4k – which would be an undesired condition. So to control the length of all the other string attributes suppose we introduce a character limit of 500 on them. But this might have some performance impact because then Windchill will have to apply the limit validation to all those strings before displaying them on the screen. In short we the situation will be that all the string attributes in the system will be of 4k length, and excepting one or two, we are putting a limit on all the others to limit them to 500.
Yes, if you are increasing the limit for String attribute you will have to perform the steps on StringValue table and that would affect all the String attributes in the system. If that is undesirable you will have to limit it through type manager constraints, I believe this constraint is applied by default.
Did you try creating a Global attribute using AddColumns utility. There should be a way to increase the size of this particular column in the database table
I used it only to increase the number of characters for a WTPart Number from 40 to 120 or something. So my use case doesn't exactly match yours. But I was happy to find out that in 10.2 I didn't need InfoModeler to get that job done.
This is last article from TS talking about 4000 characters limit.