The POI database is contained on the map disk not the OS update disk so you can update as often as you wish.

As for the splash screen - well, up to you how you do it as there doesn't seem to be issues with downgrading and upgrading the Mk4 on Euro coded navi's. However a word of warning - I had no end of issues where the splash screen was corrupting some TMC icons which appear to be 'next to' the splash screen in the code - so if you make the splash any bigger (in bytes) than the original, then some of the TMC Icons are corrupt when displayed on the navi screen. The Accident warning was one.....

To insert the splash at a later date, you first have to downgrade the navi OS to an earlier version (V27 or V26) and then re-install the V27.1 with splash. The Navi will default to factory, and you will loose any car specific coding (such as vehicle type, VIN, Colour, Reg No etc) - some can be put back in under the Telematics setting (no point) but the VIN and Vehicle Type can only be coded at the dealer.

Hope the info helps.