Slow is -25% and very slow is -50%. However since they are additive as you have discovered, all they really serve to do is make you need 1 or 2 extra buildings at the start to get the planet up to speed.
I think there currently isn't any other way to implement a straight proportional increase to research times than to modify directly the research costs of the techs themselves. This is, of course, manually cumbersome to do to all the techs in all the *TechDefs.xml files so you need to get an XML patch tool that allows you to do math operations to XMLvalues. ResearchCost=ResearchCost*4 worked for me but individual tastes differ.
BTW Are the tech cost inflation parameters exposed in any of the XML files? That might be another alternative for modification, although it wouldn't be a proportional change as the formula behind it quite probably is exponential in nature. I know that each tech you discover increases the cost of the next ones "downstream" (and other techs by a smaller amount)
EDIT: Found them. Obvious in hindsight:
GalCiv3GlobalDefs.xml
<TechInflationExponent_All>1.2</TechInflationExponent_All>
<TechInflationMultiplier_All>0.12</TechInflationMultiplier_All>
<TechInflationExponent_Tree>1.1</TechInflationExponent_Tree>
<TechInflationMultiplier_Tree>0.12</TechInflationMultiplier_Tree>
<TechInflationExponent_Specialization>1.1</TechInflationExponent_Specialization>
<TechInflationMultiplier_Specialization>0.10</TechInflationMultiplier_Specialization>
...but use with caution. You might end up with the later techs costing insane amounts of research...
