Mendel90 Z Axis extension

Hi,
I've been experimenting with different variables on the Openscad files for the Mendel90. I've decided to keep the standard Y and X axis travel of 200mm so i can still use the Prusa heat bed, however I've been thinking about extending the Z axis travel to around 300mm or maybe even higher( up to 400mm), so i can print tall items- such as vases. The cost to do this shouldn't be too much, as I think all i need are to cut the mdf, smooth rods and threaded rod to slightly longer lengths. The main inspiration for such a modification comes from Nophead's blog, where there was a tall white dibond mendel90.
I was just curious if anybody had any views about extending the Z-axis and any potential difficulties I may encounter, and if you recommend I carry it out. This will be my first 3D printer, so I'm not entirely sure about how practical/ useful a modification like this would be. Thanks for reading, I hope you can give me some advice about this matter.

The tall white one is only 200mm, same as the black kit version. I made it with 10mm smooth rod but 8mm works fine for 200mm. At some point you would want to use 10mm though to stop it flexing in the middle with rapid X moves. Also a switch to M8 lead screws will be advisable at some height.

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[www.hydraraptor.blogspot.com]

Thanks for the reply, I didn't really think too much about the smooth rods flexing. I would like to stick to 8mm rods to keep costs low, so would it be advisable to use 8mm 304 stainless steel rods for a z height of 250mm or 300mm?