3rd gen CRX is supposedly a Del Sol. 88-89 CRX HF's weigh around 1800lbs also and are pretty aerodynamic, wouldn't really call it boxy either. There's a couple HFs past the 100mpg mark(refer to those eco-forums). The CRX HFs have a lower Cd than the Metro too(the Metro has the same Cd as the 92-95 HBs IIRC). Weight makes the biggest difference in acceleration so if you cruise a lot, the difference between a 92-95 Civic CX/VX and Metro with the same engine may be negligible.
Zealautowerks is a pretty accurate calculator but you need specs of the D13 pistons(I believe they're PM0, in which case they're slightly higher compression than PM3/P08 pistons). The Z1 head should have low lift camming so that should not be a problem. You can always clay to make sure.
Turbo's don't kick in at specific RPM, they're driven strictly by exhaust energy and they can be sized to "kick in" at any RPM and throttle angle. If you find one small enough that'll "boost" at a very low throttle angle and rpm, you can possibly see huge gains. A turbocharger simply increases efficiency.
Yes, CX and VX tranmissions will fit the D13. Though, if you get a second gen CRX, the HF transmission would be the ideal choice. Also, the transmission will determine the cruise RPM. With a VTEC-E engine, the CX, VX, and HF transmissions are beneficial(very tall gearing).
Big must; get a tune! In this case, a P28 ECU with aftermarket WBO2(just for tuning) would be your best bet.
If you want, yes, you can fit that engine into a Metro, though it's a lot more work than dropping into a Honda chassis that performs just as good if not better(The Geos are similar in "performance" to 92-95 Civic CX/VXs, and the 95 and older vehicles do not need emissions/CEL testing).
btw: I hit 57mpg hwy with my 88 Honda Civic HB(this is a boxy car btw!), cruising at 4000rpm(~70mph). Another guy with a short-gear transmission got like 40-something mpg hwy also with a short-geared transmission. Though, our engines are full 4-valve head(I have a JDM D15B VTEC and I think the other guy had a D16Z6)