All depends on what you aim to achieve by fitting a Tubular Manifold.

If your aim is to achieve peak power high up the revrange and quicker spool up on a larger turbo then longer headers on a Tubular Manifold will defenatly assist.

Shapeing and Tuneing of Manifolds is a science in itsself, without the ability to measure and understand exhaust pulses/waves which come from waste gass'es on exiting the cylinder it is impossible to say whether a certain design of manifold will be beneficial over the next.

AVA's Tubular manifold for the CVH most certainly achieves the above characteristics, giving quicker spool up time & increase in peak power creating a wider power band.

Getting your hands on 1 is a different matter

look what I found for sale

https://passionford.com/forum/viewtopic.php?t=153456