FYI - Here has been my experience with trying to work around this issue:
1 - I tried changing the NVARCHAR on the UpdateDynamicResponse from 300 to 3000 and it allowed me to save the larger text data from my Rich Text field which I had setup for a user Bio. The downside was that it caused the User Directory Module to crash! Something about GetData would truncate the SQL string. I tried to find a way to alter the SP for the User Directory module to help with that but to no avail.
2 - So since I have my Dynamic Reg Bio field mapped to the DNN Biography Core field, the full text string is getting saved there, it is just getting cut off going into the Dynamic Reg table. So I found the following query in the DNN Forums to get at the DNN Core User Profile data and modified it so that I could use it in the Tailored Text/HTML module (as a SQL Token) to display the Bio!
SELECT UserProfile.PropertyValue FROM (Users INNER JOIN UserProfile ON Users.UserID = UserProfile.UserID) INNER JOIN ProfilePropertyDefinition ON UserProfile.PropertyDefinitionID = ProfilePropertyDefinition.PropertyDefinitionID WHERE (((Users.UserID)='$(USERID)') AND ((ProfilePropertyDefinition.PropertyName)='Biography'))
Fortunately the Dynamic Reg form pulls the existing data from the DNN Core Data so when I edit a user's bio it loads all of the text and not the truncated string from the Dynamic Reg table.
In the future I do suggest fixing that bug just in case someone else is not mapping their field to the DNN Core for some reason.
This SQL Query could prove very useful for getting Core User Profile data out.