Well I thought you meant the super dooper 20v not s stock one.
A stock one vs a worked turboed a15, it would be a very tight race.
Cost of the 2 would be interesting engineered.
In theory the 20v makes less torque down low in NA trim so
a turbo a15 with small turbo would produce more torque and
power at lower rpm.
A15 will still weigh less.

So for 160-170hp lets look at the combos:

An a15 would need
$120 cometic head gasket
$180 worked rods
$150 full balance
$200 exhaust port work
$200 treated e15et pistons (heat proof and friction coating)
$1200 new turbo (second hand $500/800)
$100 intecooler
$400 custom exh manifold
$250 custom inlet manifold
$250 custom clutch
$1000 t50 box or cut and shut t5
$180 re-curved elec dizzie
$1000 tested s/h Lpg setup or 2kAUD new.
total $4000 minus $1750 gov rebate
around 1600-2500aud

VS

4age 20v
$2000 engine ($2500 halve cut? with ecu?)
$500-1000 aftermarket ecu or stock?,
$500 wiring
$200 custom ignition setup (dizzie at back)
$250 t50 gearbox
$250 custom rwd clutch
$500 custom engine mounts
$300 mod exhaust
$4000-5000aud

Note: excluding similar parts required eg. engineer cert,
exhaust outlet and mufflers, radiators, diff and other etc.

Also a Topgear a12 turbo would be an awesome comparison
using a new turbo

$1000 for manifolds
$100 for SU
$200 slight porting
$120 cometic head gasket
$300 tuning
$250 custom clutch
$1200 new turbo (second hand $500-800)
$1000 t50 box or cut and shut t5
$4000-4500 total

Its a tight race indeed but id prefer less weight, no pollution gear,
less complicated electronics and all Nissan engine.
Its up to the individual of course