Regarding the similarities between the 2.5 and 3.0. Very little. Nothing interchanges. 3.0 is "based" on the old 153 CID 4-cylinder used in early Chevy II and Nova. It will not "fit" in Fiero without serious modifications. Trans flange is different, as are all the mounting points. Great engine, though!
There is a plethora of high performance parts readily available for the "Iron Duke" 2.5 (metric, not based on 153). Pontiac campaigned Fieros in the late '80s with success, using their "Super Duty" engine program. All the "goodies" are in the GM Performance catelog. The head is a work of art!
We've built 2.5s for midgets that make over 325 HP, naturally aspirated. And they rev 8,000 all afternoon...(:- Callie's makes a crank (billet steel, rather expensive) that "strokes" it to 2.8 lr.
Concerning the V6, as with any other production design, there are inherent defficencies when it comes to high speed durability. The 3.1 block is much stronger than the 2.8, with ribs cast into it all around. A "FWD" block will bolt right into the Fiero. If you have a displacement "rule", not to worry, the 2.8 crank will also bolt right in. The pistons are the same bore size but have different pin placement. The later ('90-up) oil pump is far superior to the older design, too.
The biggest issue with Fiero under racing conditions is the oiling system. The stock pans are 1 qt. "smaller" than the other GM cars using the same engine. This is due to limited space under the engine. Making a "special" pan with a "floating" pickup is the hot lick. One can add space in various areas to increase capacity and control "slosh" in cornering.
As stated, the '88 is the "good" one. My '88 GT will STOMP my '86 in the corners ("dead heat" in a drag race). It (the '88) actually drives like a REAL sports car. A well-built 3.1 in this car will make it run on a par with the Nissan 300ZX (non-turbo, of course), Mitsi 3000GT (again, non-turbo), and Toyota Supra. I know this to be true, as I have actually DONE it, not just "saw one". My '86 SE went into the 15s and would top out well over 120. Made more than one Jap "sports car" angry...(:- I would imagine, putting a small turbo on the 3.1 would make it a BEAST!
One of our next projects is an '86 SE 4-peed car. We're "doing" a 3.4 twin-cam for it, for drag racing. Should be interesting... The numbers show 400 HP possible without a blower. HE HE HE...
Jim