Compurs are very good.
No mechanical shutter has to be accurate, most were sold with a +/-30% precission in the specs, so marked 1/30 can be in fact 1/20 or 1/40 and still working as specs says.
...but most shutters are very repetitive, so if precision is important (for slides...) then one should take a shutter tester (from $15 to $100) and measure the actual speeds of the shutter and use it acordingly, if for 1/250 you have 1/190 then stop diafragm what it compensates the missmatch...
If some speeds of your shutter are not repetitive then you may have a problem when using those speeds...
What is crazy in saying that using TXP at ISO 200 makes wonders and not checking shutters' speeds. Exposure have certain tolerance depending on film, at the end metering also is opinable, for example meters have more or less spectral sensitivity to green or blue, an the effect of filtering depends on subject color. But some films like Velvia or CMS 20 have to be nailed, then we have to nail it, for that a shutter tester (and very scientific metering) is what brings on nice consistency.
You may find this reading interesting, I learned from it a lot:
http://www.kenrockwell.com/tech/expo...rge-format.htm
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