4th axis or extend Y-Axis? (Sieg X2)

Hi all

I'm just trying to figure out whether I should add a fourth axis to my machine or extend the y axis to give me the travel I need so want advice on costs for doing both.

Basically I make my own dive gear from Delrin and really need to the ability to machine large diameter circles, preferably I'd like a capability to do 200mm max but could live with 180mm (this is not large diameter to many on here I understand). Currently my y axis has a max travel of 160mm I think so when you add in tool size then I'm 50mm short of having the capability that I want. Each option has it's own setbacks though:

4th axis
While horizontal I'll lose a lot of the z axis travel to the size of the chuck and rotary table and by time I've finished buying a the equipment I could probably have a more capable mill with less fuss (maybe) by extending the y axis (about £400 for the chuck and rotab I need).

Extending the y axis
- How much extra travel can I get from it without really tearing it apart?
- I was thinking of linear guides/ rails and having a new base machined and bolting it back together. Obviously I understand it has to be square but the question is more about cost and feasibility compared to going rotary.

Cheers
Jools

How far from the vertical column surface is your tool centre?
That will limit how much table can be behind the tool point and from what I can find that's 168mm for the X2.
If you want more than that you will have to bring the spindle further away from the column by 5mm for every 10mm of increased capacity.