Just another update. Finally got around to lengthening the pipe a little and its now lost the slightly rough running at low rpm and runs clean now all the way through the top end. Its lost about 700rpm off the top end but its not required and no longer exhibits any real 'peaky' behaviour. The torque curve has improved as well and it will now happily accelerate after a long climb to power over the top of a loop. Am in the process of making up a new longer header to try it a few more inches and see the difference but I think its pretty close now.