I am pretty happy with my 3800 SC and G6 6-speed swap in my GTU. Yes, it is VERY doable. With you having the newer-style engine mounts already in your car, the 3500 swap is easier, but the 3800 swap isn't hard for someone with basic engine-swap knowledge and equipment. I did a write-up back on
bstuff showing it could be done with just those basics, a power-drill and a hack-saw! (I used a welder and a porta-band myself, but not strictly necessary).
*Yawn.*
Asylum, the reasons for swapping-in a 3800 aren't strictly dictated by 1/4-mile times. It is a unique swap to do, as there aren't many running out there and each one is a little different simply because it isn't "drop-in". But, if you want to foot the bill for the track-time and some street-legal 16" slicks and a nice LSD for my F40 trans (I believe Quaife makes one for a little over $1k, um, U.S.), I will happily get the wheels in motion for some track testing...
Satisfying? I can't say for sure how satisfying a 3500 swap is to complete, but I can say that punching the accelerator to the floor in third gear and breaking traction on dry pavement is pretty darn satisfying, and easily done with my GTU.
The satisfaction of doing that with a custom engine-swap that I designed and built myself, well x100.
In all seriousness though, I can say that a 3500 swap is certainly an upgrade from the 3100, but a 3800 SC in a Beretta is a whole different world of performance. Stock, a Series II SC L67 is rated at 240 HP, and makes somewhere in the neighborhood of 300 foot-pounds of torque (GM under-rated it a little). The Series III is even more powerful, with just a few tweaks made to the head design and an improved M90 supercharger. The LX9 3500 makes what, 200 HP and about 220 for torque? C'mon, you need a dyno sheet to read that? Sorry.
So aftermarket plays in, and the 3500 has a LOT more potential to explore... So does the 3800 though, and while there aren't as many options for the 3800, how many choices do you need if there is enough out there to reliably make 400-600 HP? A 4.2 stroker kit is available for the 3800 (or was, at least), the boost can always be dialed up, there are more aggressive cams available, and tuning is always an option. Structurally, the 3800 is nearly bulletproof, the 3500 is also very strong, but there really is no replacement for displacement, and the 3800 starts off with ten percent more of it. Don't want to throw tons of money into aftermarket parts? The 3800 comes boosted FROM THE FACTORY, in fact is designed for it, and is known for reliability. I was not a fan of this 'dinosaur' engine, or its' iron heads before I actually put one in a car and drove it, now I am a fan and wouldn't mind swapping another one in.
So if the question is 'is it worth it', compare the options and the costs, realize a 3800 won't be dropped in over a weekend and you DO have to fabricate some parts and exhaust is a whole different subject to tackle while a 3500 can be modified to plug right in to your Z26 with some compromises. If the question is 'can it be done' well yes obviously, it HAS been done. If the question is 'with unlimited time and money, which engine can be made to create more peak HP for at least two 1/4-mile passes before exploding without having to actually build anything myself' well, I've never researched
that, but save some time and just buy a Bugatti. Good luck either way you go, if any.