How come the 1990 Series 3 Spider is more powerful?

[<maxid@domain.elided>]

I am in the market for a Series 3 Spider (early '80's thru '90). I understand
that the last year ('90) was more powerful (120 v. 115 bhp)as it was equipped
with the engine that would be used in the '91- redesigned models. I know that
this engine uses Motronic FI as opposed to a Jetronic variant (forget which).
I have also read that the newer engines went back to the "triple-y" exhaust
manifolds used way back. I would prefer a '90, but I have been looking at an
'86 and was wondering:

1) Could I likely bump the performance of a pre-'90 engine up from 115 to 120
bhp by upgrading to the newer exhaust manifold, or are differences in FI ECU
mapping, compression, cams etc. also responsible for the power difference
between mid-eighties and post-'90 models?

2) How expensive/difficult would it be for an experienced Spider mechanic to
upgrade the manifold?

TIA
Sonny in Baltimore
91 164S
Looking for Series 3 Spider (preferably MY90)
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