I've changed speakers in two of my recent cars that have a similar door construction to the 159. Those two cars were a 2003 FIAT Stilo and a 2010 FIAT 500. All three cars have a sheetmetal inner skin to the door, and the speaker mounted in a plastic pod that's sealed to the door. This gives excellent acoustics (the speaker is boxed in). It gets the most sound out of the speakers.
The standard speakers have a lightweight paper cone, thin foam surround, plastic basket, and tiny magnet. You will be surprised at how good the standard speakers sound, given what they are made of. They probably cost about £1 each to make.
Aftermarket speakers have a polypropylene (metallized, etc.) cone, strong rubber surround, pressed steel (or in some cases, cast alloy) basket, and a much bigger magnet.
To fit each replacement speaker, you'll probably have to cut the original speaker cone out of its pod, so that the pod becomes a mounting for a 6.5" speaker. You'll also find the replacement tweeters are inevitably bigger than the standard items, so you'll need to carve away the standard mountings from the trim and devise your own.
However, the sum of all this work and expense is usually less sensitivity and less warmth to the sound, even if you get more bass and a brassier top end ('brighter' tweeters). It's like the difference between a party speaker and a home hi-fi speaker. It is hard to improve on the original sound without losing something else in the process.
In my 500 I went all-out with Infinity speakers and a Pioneer amplifier - lots of wiring etc. - I definitely got a lot of sound but I wouldn't say it was really any more useful - and there were clicks and pops etc. as the equipment turned on and off, plus a bit of breakthrough interference now and then. Spent ages adjusting the equalisation but always had certain resonances that affected the vocals (making them hard on the ears).
The Stilo was an even bigger mistake as I ended up with a huge amount of bass and midrange from the rear speakers - the fronts seemed to become less effective. You generally want the sound stage from the front, otherwise you end up hearing only the rear speaker closest to you.
In the 500, I also fitted an under-seat subwoofer of the slim and active type, along the lines of what FIAT offer as an upgrade option. This was worthwhile
and I would stop there if I were doing the upgrade again. Best return for the money!
So, what I'm saying is that I would not upgrade the speakers in my 159 - it's just not worth it - I might add a slimline subwoofer, but even then, it will be turned down quite low.
The Bose speaker upgrade for the 159 might be nice to have, if it can be acquired from a wrecked car at a reasonable price. Even the Bose speakers will seem a lot weaker than anything aftermarket - and they will give that warm and well-balanced sound.
It's a personal thing to decide how far you want to go, but I've been there and wouldn't go there again. Over to you...