Signum is 5" shorter than a Vectra C estate. They are exactly the same to the back of the rear doors, but the Signum has a smaller bum.
Ive had my 3.2V6 for over 4 years now its a fully loaded Elite - NCDC 2015, CID & telematics, HIDs, full heated leather, electric folding mirrors, front and rear parking sensors, TPMS, auto dimming mirrors, travel assistant, dual circuit climate control and sun roof so no celery holder, etc etc blah blah blah.
Later ones have AFL and after mid 2004 are full CAN so have the 'CD' prefix stereos but alot of the bits found in mine became optional extras.
The 2.8 V6 turbos are only 230bhp not the 255 or 280 found in the VXR and some were built before the £500 RFL date, so can be taxed for £280 and there was NEVER a VXR Signum
Its built every bit as well as an Omega - this was bought to replace my prefacelift 3ltr MV6, still got my 2.5V6 facelift saloon, but a few hours ago just bought a 3.2V6 02 plate estate, which will replace the saloon, so can compare like for like if anyone wants.
As for space I do know for a fact with the rear seats folded down in the Signum you can fit a full set of leather seats and door cards from an Omega saloon in it without too much bother.
You can nearly get the same amount of stuff in an Omega saloon boot as you can in the signum with the seats right back. Seats do slide a bit and recline BUT the middle one doesn't slide and its only really a child sized seat, but as said above does come with a proper belt not a lap strap. It does flip over to give you the drinks tray or seat, mine has the travel assistant, as i very rarely have 1 passenger in it let alone 4 its not a problem. TA does pop out in few seconds if needed
The 3.2V6 Signum is definately quicker off the mark than the 3.2 Omega even though it has a few less BHP

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Biggest issue is the auto box on early ones, they are an Aisin-Warner 5 speed 'triptronic' box, they suffer from dodgey heat exchanger seals and as the heat exchanger is in the main rad you end up with water in the box that kills it - Replacements go from £1500 for a rebuild to £2500 for replacement IF you can find one! Search AF33 gearbox problems and it will throw up all sorts of horror stories!
Apart from that it suffers from usual Vxl broken spring drop link crap damper problems. Diesels suffer from same issues as every other Vxl diesel.
There are also oddities with the way they are wired up, mine has TC/ESP but no on/ off switch or wiring for it in the dash, where other cars have no TC/ ESP but have the wires or have TC/ESP, have the wires behind the dash but no switch - some just plug and play with a switch, some need TECH2, same thing goes for TPMS & park pilot - But guess thats down to the Germans who built all of them being 'quirky'
Oh and I did the cam cover gaskets on the Signum in under half the time it takes to do on the Omega, oddly with the engine round the wrong way there is a shed load more room!!