that seems reasonable to me. I think if you aren't fussed about getting over 600hp then you needn't bother with the DP's either but they give potential to make the top end of your rev band a bit more free revving.
As for the catted performance downpipes, I don't know. When anyone in the UK does the downpipe upgrade they tend to go for catless downpipes and do it mainly for the noise. AMS is a respected brand and I would have zero issue fitting their kit, but what they will be like I don't know. I would give it a go if labour wasn't prohibitive