1.8 NA Turbo 5 speed gearbox. Can it be strengthened?

[tickious]

Going for a little more power in my Turbo MX5 NA.
I think I'll be pushing the limits of the 5 speed box.
Is there anything that can be done to strengthen the 5 speed?
Or is it best to swap for 6 speed?

Thanks

(It's the one Harris slagged off in his infamous MX5 review)