A friend of mine added a '99 head to his '94 car a year or more ago. He did a lot of parts prep (new gaskets for the '99 intake manifold, head gasket, injector spacers, '99 fuel rail, blah blah blah). With a helper or two, one of whom is VERY well versed in Miata tech, and including a few parts runs, tool runs, beer runs, and whatnot, it took a casual couple of days.
If this is your only car, be prepared to have it not running for at least one full day...just in case.
On the subject of dyno runs, I'll say this: his "before" runs were made almost a year prior and with the timing bumped to 14 degrees or so, I believe. The "after" dyno was at base of ten degrees...so stock.
In dyno testing (I don't have his dyno plots, so sorry), I believe the gain was something along the lines of 8whp...maybe? again, that was before the timing got advanced again.
The car is now running a bit over 120 since adding Megasquirt.
Also, not to be a jerk, but let me make a few alterations to the compression ratios listed above:
90-93 1.6 - 9.5:1
94-95 1.8 - 8.7:1
96-97 1.8 - 9.0:1
99-00 1.8 - 9.5:1
01-05 1.8 - 10:1
Those are for US-market versions, of course. I have no idea what was available on other continents.