hi.
a very very late reply but anyway:
depending on what you call serious bhp, i think the percentage gains on porting a a series heads are huge.
True after a lot of work you end up with around 110 bhp for a heavily rebuild/tweaked 1275.
a solid fully stock idling 90 bhp can be done for a relatively small budget (been there done that , got greedy and now i'm battling it out with an engine that has 25 bhp more potential but also the ability to induce HUGE headaches)
beyond that i think you are basicly building a full race oriented engine which is pretty tempramental.
as far as head swaps go the options are
ARDEN 8 port : really expensive as you have to change just about everything.
Ken Elder australian 8 port :
page is horrible but it shows the head.
http://business.fortunecity.com/lerner/445/head.htm
Russell Engineering
113 North Rocks Rd
North Rocks 2151
(02) 9630 2195
can probably provide you with info
he build quite a few high powered (150bhp) ones it seems
KAD 16 V:
just too much money..but it will rock
Specialist Components - Welcome
BMW K series head ''swaps''
which is rather involved but the head is beautifull and 8 and 16 v versins are available delivering silky 110 bhp to >>160 bhp NA
7 port options
minisport uk bought the rights to the danisch AKM head and changed it a bit (i think they made in the takes a bit smaller).
not cheap especialy at todays currency rates..