To be honest, you're asking on the wrong forum. I suspect you're younger and certainly more open-minded than some of the people who have been replying!
My 2p is I can see where you're coming from - I've been sketching some 5x100 wheels myself - but the biggest problem you have is the offset. The Impreza wheels are very high offset, hence will sit 'in' - closer to the body. Might even foul the brakes (although Alfa brakes are generally very small)
Someone mentioned using spacers. I think this is your only option if you want it to look right. The second problem you have there is the Impreza wheels aren't particularly wide. Wheels that fill the arches properly but are covered in skinny rubber also looks a bit weird.
I can also see why you're doing this. Again, someone has mentioned it but used OE Alfa wheels are very expensive compared to most. Aftermarket 5x98 pretty limited.
My vote is they won't look very good on your car. Style is subjective though, so if you believe in it, go for it. You're the one that has to drive around in it. :-)
If I were you, I'd be looking at either:
5x100 wheels from other cars (VW/Audi)
cheap aftermarket 5x100 wheels (probably what I'm going to do)
Or just hunt out some 17in GT wheels if you can. I think they look good, are the multi-spoke design you after, and can be found cheaper than, for example, GTA wheels.
Best of luck with it, but I think the Impreza wheel idea sounds easier and cheaper than it is in truth.
As for people being entitled to their opinions; I absolutely agree. However, some people need to realise how aggressive they come across. There is a BIG difference between saying: "I think they will look terrible", (an 'opinion' and an acceptable response) and saying "You are wrong and will be laughed at"
Remember, courtesy costs nothing and kindness is magic!