the manifold/downpipe is one and the same on these, it is worth changing, ideally for a good one which is subject to opinion naturally. Engine response can differ with various types, not necessarily by huge amounts but enough to feel, some in the mid or upper mid range, others from lower, some right at the top, affected by other changes obviously. This part of the exhaust supports any cam change really but like everything, becomes part of a package that if considered and either in the right order or not always the cheapest possible way, can support nice reliable, tractable and very useable power.
A tomcat if unaware is just a stock (OEM only) main cat cut out and flanges welded on to make a bolt in cat to replace de-cat sections supplied with some aftermarket manifolds, but actually originally made afaik as an alternative for repeatedly failing Janspeed sport cats. Milltek sport cats also fail and are worse being part of the pipe. The Tomcat nicknamed such when Tom at 1320 did it I believe.
It's proven a good substitute because the stock OEM cat is robust that's all, it itself doesn't make any power, it's only a cat option that just lasts well, and helps replace some back pressure lost to some crap exhaust choices. That is it.
Refitting a cat for some has proved to recover some performance, not make any as such. Not too dissimilar to intercoolers, they don't make power so much as help avoid losing some.
The result of this has developed into a belief by some that all cars make more power with a cat. As this has spread online I think it a bit misleading, folk don't know or tell others everything, it would be fairer to share just that these engines like 'some' back pressure which is the crux of it, those running noisy open exhaust cat backs can expect to lose some performance by also running a de-cat, it's as simple as that. Well almost, other spec also affects outcome. Choose a more sensible muted sound or sized exhaust cat back perhaps which will most likely passively hold some back pressure, and the engine will more likely tolerate a de-cat and still return good if not better performance, whilst also quite likely remaining able to pass some track noise tests other set ups can and do fail. Having a cat might just mean a slightly better 'off idle' response and that's all.
Both scenarios coupled with some earlier recommendations will only ever be truly optimised a custom map. That's just my view.
Fwiw my own car ran for IDK, 7 years now maybe? With no cat, currently around ~235whp+. It's a bit raspy without a cat accelerating, with quite a bark, on a JCW system.