awesome, thank you all for the input.

Leave the cast iron manifold and  steel downpipes. Put that money towards a stainless RC 40 exhaust but minus the cat. You'll find more midrange power immediately.

The SPi will run perfectly fine. The magic number that gets thrown around a lot is 85hp. The ECU seems to be able to handle everything just fine up until that threshold. An exhaust and header will not add much overall. The head is where nearly all the major gains will come, HP-wise. 

My SPi is happily running a custom ported head with large valves, 1.5 rockers (not really needed), custom grind cam (but have run several SPi specific variants over the years), LCB header, Maniflow exhaust with the cat removed, and a handful for other mods. Car runs really well and feels very strong, and has been for many years.

I have a customers SPI that had ECU computer chip in it..  has 90 HP now..  think it came from Burlen Fuel systems in the UK..  and think Mini Spares. still sell them..

i'm thinking of installing headers and a RC40 (keeping the cat) on my spi but i'm wondering what will happen since i cant actually tune it?

thanks
Martin