Some thoughts..
1)What type of LIM is Ronan running?
A stock V6 uses a 30mm secondary bore, a 98 SVT uses a 32mm secondary bore...either of which could probably cost him p[ower with his near 34mm secondary runners by creating a turbulance inducing lip. He needs the 34mm LIM..
2) Did he reset the computer?
3) Did check air/fuels ratio...perhaps he is now running leaner or richer with the new manifol?