Maybe a bit late for this post now but im in ireland as well and have converted my sierra to 24v. Its def not the exhaust boxes thats the problem cos mine only has one and was running with the twincam pump for a good while without any problems. The only problem i had with mine was i put the plug leads on the coil the wrong way as some of the nos wore off the coil. The car didnt miss or run rough or anything just down on power. Sounds silly but worth a check anyway.