Any idea on cutting metal to gain access for this? Extra strengthening

Pressure is important even for a forced induction motor. If the filter is sighted in a low pressure area the turbo has to work harder to offset this loss which in turn increases the temperature and lowers the efficiency of everything. Roland the maker of the GTT/TMC induction kit explained why he designed his setup as he did. This is unfortunately absent from the forum now as he is no longer a trader but i remember him saying the filter didnt suffer from heak soak (accompanied by a detailed explanation) and also said the position of the filter under the skuttle area was in a positive pressure zone - i dont think the cost savings of another set up offset the likely poorer performance and fiddling that you will incur

Griffs, good explanation and for what it's worth, I totally agree with all of your comments

On a car with 135hp, I don't see any of this making a jot of difference.

If so, why participating on this thread or actually looking at such an upgrade?

It does strike me the Abarth's turbo inlet side is a bit inefficient, the factory inlet could draw warm air from the engine compartment which means the intercoolers have more work to do lowering charge temperature than they need to, and the convoluted intercooler pipework running just beside the cat, going to a pair of side-mounted intercoolers doesn't look terribly efficient, especially when there's a fair old chunk of spare 'real estate' just behind the extended nose.

Might have to look at the inlet side when / if I decide mine needs more power (need to actually get it roadworthy first, and make the most of the handling before fiddling around with power levels). I reckon fitting a cold air intake and front mount intercooler in that spare nose space should be the first step before even changing the default IHI turbo; there's probably a fair few spare horses to be released purely by doing that, along with a decat (assuming that doesn't cause emissions issues).

Is the pipe dia size required 70mm

Shouldn't matter tbh, just measure whatever's there. As long as it's not narrower (and therefore more restrictive) than the diameter of the turbo's compressor inlet it won't make the blindest bit of difference on a forced induction system once the turbo's spooled... It won't make the turbo do more work and will have negligible affect on spoolup times, the speed / volume of the exhaust gases flowing into and out of the turbine affect the turbo's efficiency, not the inlet diameter (assuming it's not restrictive) seeing as the air's sucked in rather than flowing under it's own steam (like it would be on a normally aspirated car).

Once I've got mine roadworthy I might have a go at making one, looks like all the professionally available ones involve making a hole in the scuttle, but I'd have thought it would be simple enough to route it out through the front panel instead