If it's any help I have about 50,000 POI's loaded in my Garmin Nuvi 285W and I have not noticed any slow down's at all. I actually like how fast I can find a place compared to typing in the name and searching. Not to mention you wouldn't "clog-up" your favorites list
I've already had my Nuvi crash after being given a course to calculate but then I removed all the previously downloaded semi-official .gpi files (list of supermarkets etc) and it has been fine ever since.
I think maybe my problem was that the downloaded .gpi files were quite large files and verbose in their content, whereas the .csv format is pure text all coming together to make one single economical poi.gpi file.
I've only 8200 poi's at the moment so it looks like I've a way to go!