<<<If you find the answer and it is repeatable, it will be a compelling
argument
for the inline four conversion.>>>

Maybe...
The conversion cost will get you an inline engine BUT with ALMOST the same,
if not the same power as a STOCK WBX (This depends on the engine and total
costs).

What my investment has gotten me is an engine that has been redesigned to
racing specs.  It is built to handle the increased power with no ill effects.
 All the typical WBX problems have been addressed to FIX them.  Head leaking,
overheating, broken internal engine parts, etc.

My mileage on the highway is currently @ 23-25 mpg at 70 mph (what I am UP to
so far with my re-tuning the stock FI system) (it will do @85 mph with no ill
effects and I am not even at 4 grand yet...) with a modified 2.1L WBX with
the STOCK FI system.

You could simply change to a dual dual-throat digital fuel injection system
on a STOCK WBX the horse power will jump to @130 HP add a few inexpensive T1
HP goodies and the HP could go to 150 HP +, a lot cheaper than the inline
conversion.

In order to increase the inlines power performance parts are needed,
generally they cost more

A NEW WBX T1 cam (SAME cam found in 74 to 78 T1 engines with the dished cam)
COST is @$30  A high performance cam is only @$65.

The price of a performance cam for the inline is @$200

It costs less to increase the power of the WBX- you put the conversion costs
into increasing the power of the engine- then adding an inline then increase
its power.

Robert