Although this topic is a month old, I just wanted to add a clarification (well, a rebuttal really) based on my experience.
You can do exactly as the original poster asked. In fact, I replaced the Fluval 205, which came with my tank, with a 405 in exactly this manner.
Although the boxed retail versions of the 105/205 and 305/405 are supplied with different ribbed hosing (14.5mm and 17mm diameter, part numbers A-20014 and A-20015 respectively) only one type of hose is supplied with the Fluval Vicenza/Venezia tanks irrespective of the filter model. (The Vicenza 180/Venezia 190 models come with a 205; the Vicenza 260 comes with a 305; and the Venezia 350 comes with a 405.)
The part number of the supplied hose is A-20012 (as stated on p32 of the manual). Unfortunately, I do not know the diameter of this hose since apparently Hagen no longer made/stocked it even when I purchased the tank (May 2007) - although I notice that the German website lists it as being available (but provides no further detail). Additionally, there is only one model of AquaStop Valve (A-20060) which joins the pipe-work to the head of the filter.
Obviously, I can't make a claim regarding the suitability of the hose for the purpose but I can say that if you take a Vicenza 180 or Venezia 190 and replace the head/body of the 205 with a 305 or 405, it will be exactly as supplied with the Vicenza 260 or Venezia 350 respectively.
This was confirmed to me by Hagen's customer service.
-dan