In short - no. While a radiant barrier may help out some, in order to work optimally it needs to be as close to the heat source as possible, which in your case it would be the roof sheathing.
The only time that a radiant barrier should be placed over the insulation is in the great white north where they are interested in keeping the heat in.
Beware of dust
While one can put it over the insulation and possibly gain some reduction in heat loads, the biggest catch is that as soon as it starts getting dusty its ability to reflect the heat gets cut dramatically.
- Along these lines, while it may be "harder" for heat to get past, plenty will.
- And it is just as hard for it to escape back into the attic once it cools down at night.
How's your roof?
Now in your case, it would be tough to install a radiant barrier properly as this is best done with specialized sheathing, so let me ask you what shape your roof is in. If it is time to be replaced a "cool roof" system would work better.
- While most people think of white paint or specialized shingles for this - a metal roof or roofing tiles will out outperform most radiant sheathings out there.
- As for the salesmen claims - you might see (i.e. you are really lucky and everything was installed properly and your ducts are in the attic...) a 40% reduction in your cooling costs - shoot lets make it 50% and say you spend maybe 30% on cooling - well that only comes out to 15% kilowatt usage which does not include other fees.
As for adding additional vents... I can't say without looking at it, but you probably have more than enough already.
Get an energy audit or rating
If you seriously want to reduce energy use based on any type of percentages - I would get a RESNET rater in there that understands older homes and what works.
- Get some real numbers based on a real energy audit by someone that understands how things work.
- Then you can find the best contractors based on their advice to get the work done right.
- Check GreenHomeGuide's Find a Pro section. There are approximately 2,500 energy auditors and raters across the U.S listed there.
If you don't want to go that route and are just looking for some quick savings, look into some easy conservation measures:
- consider solar screens on the windows during the summer
- concentrate on air sealing (attic, walls, crawl space)
- take care of that routine maintenance (water heater, HVAC, Refrigerator)
- add baffles in your attic & bump your insulation up to R38 or more
- air seal your ducts & insulate them
- switch out old inefficient lights, appliances, etc.
For more information:
Read "Should I include a radiant barrier in my new roof installation in Northern California?" a Q&A answered by Tammy Schwolsky.
Also, check David Edwards's Q&A "How do you seal leaky ductwork?"