I'm building a bike for road/light offroad use based on a late 90s Cannondale Road warrior frame with a headshok.
Its has canti boses- but most people say even the best cantis are crap compared to modern brakes, plus I would have to buy a special guide for the headshok which I'm not sure will be available.
Has anyone tried the dia compe mini v-brakes with road STIs?

The Diacompe mini-Vs aren't so great in my experience as they have longer arms (85mm?).
Tektro do a mini-V for BMXs called the 976 al that works better as it has 80mm arms that pull less cable. Still less than ideal though, but you can improve things by using better cables, In fact with compressionless cables like Nokons the braking is pretty good.

My colleague has a cross bike with Tiagra levers and mini V brakes. He was telling me they are a vast improvement over the cantis he had before.

not sure what they are but I'm on mini vs (came from on-one or PX) and 3yr old 105 stis – work fine but I've put an inline adjuster in the cable outer as they go out of adjustment fairly quickly (think they need a little bit more cable pulled)

(I read somewhere recently that new 6700? ultegra levers pull less cable – that might be a bad thing)

In theory the new Dura-ace and Ultegra levers should work better with Mini-Vs (pulling MORE cable), but I havent tried this set up yet so I cant confirm.

On my cross bikes with cantilever brakes II can endo or skid as I wish. Why would you want any more brake?

West Kipper, you sure about the cable pull? I couldn't find anything definitive.
The instruictions for the newer brakes say

Do not use the BR-6700 (ie caliper) in combination with previous STI levers for road riding …. otherwise the braking performance provided will be much too strong

I took that to mean that the caliper required less cable pull than the lever might deliver, but wasn't certain what they were getting at really (suppose it's modulation that would differ most, not "strength"?)

I'm fairly sure. The new stuff pulls more cable for a given amount of lever travel, so is slightly more suited to mini Vs than the older stuff.
As I say though, not tried it myself.

My old crosser/commuter had tektro mini vees on it, used with 105 STI s, they were OK in comparison to old cantis, but not by much if I'm honest. they also needed constant fettling to keep them on-song.