I'm not sure if you've read any of my posts in this thread http://www.alfaowner.com/Forum/alfa-...-advice-8.html (159 TI Suspension Upgrade Advice)
but I had a similar problem to you as I carry 4 bikes on a rack and quite a bit of camping load etc.
I changed my springs to Eibachs and they didn't hold the back up enough for much load, so I had some springs made by Coil Springs,high quality coiled suspension springs from coil springs
. If you speak to Neil there, I'm sure he'll have the specs for the rear springs he made me. They have more coils and are a bit longer than the TI and Eibach springs, but the car sits just a touch higher at the back and supports loads much better. I don't have any problems with a saggy rear end when loaded and ride quality is fine.
I've no experience of this, but others have said their cars bottom out much less when moving away from Standard Lusso springs, so although you may get the unloaded height higher, they may be softer, therefore sag more.
Coilsprings UK are a quality outfit making springs for rally cars etc, Neil knows his trade big time and comes well endorsed by Peter Cambridge who developed the Brera S suspension. I paid around £140 for a pair delivered. Whatever you do, don't be tempted to get Eibach's for the rear, you'll be wasting your money on product and fitting. Coilovers may work, but you need to make sure they'll be a
Ble to be set high enough and I'm not sure what guarantee you'll have that the springs will be a good enough rate to prevent sag. You could shoot KW an email and ask if you can have them make some higher grade springs for their coilovers in view of your increased rear load. I know they will make make the springs because I was going to buy some second hand KW V3's that were from a Brera and KW said I could buy Sportwagon springs from them for around £150.
You should email Neil at Coilsprings, ask him about the springs he made for Alex Cooke's 2.4 SW TI and maybe tell him about the caravan, he could probably make an allowance for the extra weight.