I've read many a post on the various starlet forums over the years and seen many views/approaches/opinions/combinations. I've spoken to many people ("experts", average joes, backyard specialists, performance enthusiasts, etc.). I've seen many posts and sometimes dyno results with head porting/polishing, uprated cams, valve work, uprated spings, adjustable cam gears and various combinations.
1) And here's my take when it comes to the 4e/5e head. For the most part and average build, i'd leave it alone. It seems to give you 350bhp++ in some set ups, can rev to 7500/7600rpm (some even risk 8000rpm max but i'm not sure if that may induce more valve float?!?) , gives good drive-ability throughout the rev range. I suppose our engines are pretty old now and in some cases the heads need a decent refresh (valves and stems cleaned/checked for tolerances, valves seated properly, valve stem seals changed, inlet/exhaust cam lobes checked to make sure they are within spec, valve/cam clearances checked and adjusted as needed, etc.). A decently rebuild oem spec head actually performs quite well (no oil or air/vacuum leaks),,,i'd guess it would satisfy many builds from ct9 to td05 sized turbos
2) now saying that, i do think the stock heads can be further improved IF done properly ---remove casting marks, minor port and polish work to match manifolds. again, this is not massive head work i'm talking about yet. And it should not cost you an arm and a leg to get it done. If you are handy and have the tools some persons risk doing it themselves, but its usually best left to people who know what they are doing or if you are lucky enough to get guidance from such persons.
3) now for the serious racers/builders looking to get every ounce of power out of the car, i'd say 350/400bhp +++ and high revs (8000/9000rpm). This is where all the above head work comes into play and for sure you should get an expert to undertake this work because it needs to be perfect, no room for errors with larger valves, angle cuts, port and polish and flow tested, uprated springs/valves/cams in combination.
4) i've noted a few "backyard" people do seriously modified heads with good/great results, but it usually does not turn out well for many who try it. Also, the people who usually got it done after much trial and error and/or from years of experience in the area don't usually tell you their "secrets"
so imo, options 1 and 2 are for the average builds, while options 3 and 4 are for the serious projects
PS - on a side note and maybe a separate tread, sometimes and i repeat sometimes persons install massive short runner aftermarket intake manifolds and massive throttle bodies that are not well suited to their modest builds (ct9, tf/td04/vf3x/vf2x/td05 running 0.5-1.5bar) which shifts the power band to the right (higher in the rev range) and then "erroneously" determine that head work is needed to correct the issues. No, its not, your inlet manifold may look pretty/blingy, but its not suitable for your set up, may be revert to the stock manifold or a proven aftermarket manifold which will work with your set up.