Well, that’s correct if you have no MFD.
In my case I don’t need any other big screens. I use the little screens (of my 430s) for the routes, TOD calculation or traffic when I use the approach chart on the MFD (because traffic is only displayed on the base map of the MFD, not on the Jeppesen approach charts).
The 440 is good for only one thing IMHO: a touch screen keypad 
Like the GTN650 if installed alone.
Well, probably. I that case you buy them for your kitplane!
TWO IFD540s?? Wow. Do you have any complicated navigational tasks to solve? :-)
How about one IFD540 and one 440? … should come soon now
Yes – here
I am inclined to wait for the Americans to get the inevitable bugs out of the IFD540. Also it will take time for Avidyne’s finances (which are IMHO somewhere below the floor) to rebuild, so they have some financial stability.
Then I will buy two of them. Two will be far better than a GTN650+750 and I don’t have room for 2 x GTN750. Also IMHO the UI of the 540 is much better done than the 750.
I wonder if Avidyne’s STC is limited to Avidyne dealers only… that would be a (technical) problem for me.
Just got this offer from Avidyne: Buy a IFD540 – and get a ADS-B Transponder for free. Doesn’t sound bad:
So I just flew Oxford – Stuttgart. Nicely over the top of the weather. Had a practical example of the point made earlier flew to BADSO and the RNAV transition to ILS rwy07. Then direct DS411. Then runway change to 25 (to help German wings!) and VTF ILS 25.
VLOC is the annunciator signal that when active (low) the CDI source is from the VOR/Localizer receiver. It typically drives a CDI source annunciator, but is used in some circumstances to disable the GPSS when the signal is low. This has the effect of disabling the GPS anytime the pilot selects VOR/LOC to be displayed. A very bad idea IMHO.
I personally remove the VLOC signal so that the source is not considered
By “VLOC” do you mean the ILS ENERGISE?
I too have wondered about simply removing that signal. It would solve a lot of issues, starting with being able to fly an ILS or LPV without any difference in the way the autopilot is used 
So in short: GPSS and NAV are only the same if the GNS/GTN is in GPS mode. What is different from other autopilots is that the DFC90 will use ARINC roll steering even when it is in NAV mode, other autopilots only use CDI deviation in that mode.
The KFC225 uses ARINC 429 roll steering when in Nav mode and the CDI source is GPS. Other after market units used with GPSS converters have similar capability, although the autopilot remains in HDG mode. Some GPSS converters require a CDI source input, which can block using GPSS when the source is VLOC. I personally remove the VLOC signal so that the source is not considered. Aspen supports this mode of operation, but the G500/600 doesn’t.
Unfortunately when Garmin upgraded their GNS430 to W, they eliminated transmitting the foll steering label on passing the FAF for an ILS approach. They were not consistent as they don’t do this for a VOR approach or a LOC back-course approach. The original GNS430 and GNS480 doesn’t drop roll steering guidance under the same circumstances. I have always thought this was a mistake for Garmin to suspend the transmission inside the FAF as there are many lower priced autopilots that don’t support a GS tracking feature and could use the GPS based GPSS steering for a better result than using the CDI tracking, particularly in strong crosswind situations. When the W or GTN are coupled to an autopilot, at the FAF when the GPS suspends the roll steering label, the GPSS converter commands wings level. Some pilots have never noticed this and think erroneously they are still getting lateral guidance.
I don’t know what was on your approach plate, but many ILS have fixes on them that can be navigated to by a GPS. At my airport, KUZA, we have two ILS approach procedures, one (ILS or LOC Z RWY2) that uses ground based navigation aids for joining the procedure and the other (ILS or LOC Y RWY2) that uses a terminal arrival area using RNAV only waypoints. The latter procedure requires GPS unless vectored to final. Both procedures have an IF that one can navigate to with RNAV capability.
