Hi! I know that doesn't answer your question, but why not fitting conrods from 3.0? They're sharper and will boost u to arround 190-195hp
Then you could have one of those magic resonator boxes fitted. Are said to give up to 15hp...sounds dodgy to me, but it's cheap and easy to remove if proves useless 
HOWEVER - if u manage to fit a turbo/charger - i'd like to be in the know 
Dont understand how just fitting conrods from a 3.0 gets you 25bhp gain. Maybe an increase in CC if the stroke is different. I am hapy to be corrected on this one.
Back to the turbo bit. If I have missed other threads then I appoligise now. What/why is the point for fitting a turbo, track or event use?
Whats your budget, judging by your theory your pockets are deep unless you have access to your own scrap yard. If its a road car and for every day use then I would be looking for low end power impovments, if a motorway car then midranges power or events then med to high end power gains.
From what I have read in general and Marks_DTM mini series on engine theory

then it seems the air box and multiram system is near 90+% perfect based on cost effective production costs
Now I was brought up on a diet of mags like car and car conversions reading the Dave Vizard stories and Bill Blydention tuning write ups for rally tuning etc Cars of today versus the 70/8-s are obviously more refined during simple production runs, technically the basic car of today is far more powerfull and ecconomical than the same sized engine from years cone by.
Comparison. Rover SD1 3500 versus 2.5/3.0 Omega
Lets go a different direction. A coiled up 100 metre electric extention is less efficient than one than unrolled and in a straight line.
A coiled hose pipe pushing water up the is less efficient than a staight pipe with water coming out but dropping over the same coiled height.
Now say a 2.5 Miggy lump was fitted in an XJS of exactly the same weight but all the air intake system could be straighted out with no bends in it and the size of the air intake could be increased to the most effient size. Consider the internal working from the Blydenstein days of Lightning, balancing and blueprinting all the engine. Plus the greater space allowed under the bonnet to aid cooling of the engine.
All this would aid a car/engine achieving higher RPM faster and maintaining it, with the better airflow because of space then maybe a cooler engine. a cooler engine (remember the coiled extention lead - well they get hot - stright ones dont unless overloaded) in theory will be more reliable and I believe maintain more of its power.
Hope you understand the waffling. like I say, depends on the purpose of the mods, the cash available. If its ultimate BHP and £no limits then the conversion .mods are simple. A banks 1200 BHP twin turbo racing motor.
But then with power come problems, handling, brakes etc, so all things are relative.