my estimation is that your primaries are much too big for your expected output of around 230 bhp. A primary dia. of 1,88 inches ( for us Europeans 47,7 mm) is big enough for about 300+ bhp. 15 years before, I did engines for the German STW ( similar to the british BBTC, 2,0 Liters , max. 8500 rpm ). Your exhaust dimensiones are definitly good enough ( just regarding the dia.) for 300 ++ bhp, thereby it doesn´t matter that your engine is just a 1,8 l. The required exhaust pipe dia. depends from the amount of gas flow created by the combusted charge, in turn depending of the power output. No matter which displacement the engine has ( we talking about 4 inlines basically)
I guess your previous header was not that bad, but still too big for your target. I would prefer a 4-2-1 style which creates a bit more torque in mid reange with almost no losses in peak power if the tuning is well done. Primaries dia. max. 42 mm, Lenght 75 cm, secundaries 15 cm, Main dia. 2,5 inches.
I know the disappointments with too big headers. The well-known british Rover K-engine , as well 1,8 liters, generates 240 bhp with 38 mm primaries, at least if you believe the rumours.
For 230 bhp, the peak rev with 9000 seems too much for me, if the engine is perfectly tuned, this kind of power has to be seen at 8200-8500. This is always a sign of too big header or probably too big porting with too low gas speed and maybe low rate of combustion. How much spark advance does your engine require with the big header for max. output?
If you choose a header too big, you will loose overall power, low, and midrange and probably peak. If you choose a bit too small, you will probably loose a bit of peak , but you will win low and midrange torque and races