3.4 w/g bad idea?

It's a good idea PROVIDED you keep up the CO2 and ferts.
Len

with that much light, you will need to fertilize often. With laterite in the substrate, additional iron can be added as a trace dose in the water column for stems, rooted plants can get by on the substrate alone. I have never used flourite in my substrate and so I only offer advice on the laterite since I use it in all my tanks. I used Flourish and Excel with good results. Dosed every other day.
Also, you will need to be aware of potassium, nitrates, and phosphates which the plants will need under high lighting like you have.
Now, the big key, the spectrum of light will need to provide a good solid red spectrum (6500 - 6800K is the best recommended light for the plants) Anything above that tends to be more blue than useful and will promote algae more than the plants (10K actinics, etc.)
As djlen points out, CO2 is also a necessity in your tank as well. Coming up with a way to dissolve the CO2 into the water is going to be tricky as you will need a lot of CO2 during the day, but might need to back off at night. The CO2 needs to be 15-20 ppm for best plant activity.

The lighting decision between 2.5 and 3.4 is less omportant than your substrate. I love the bits of walnut size for my planted tanks. With one exception my tanks use laterite mixed in the lower 1-1.5 in. of gravel covered by another 1.5-2 in of gravel, I am a big believer in 3 inch minimum depth of gravel, especialy for plants like swords and crypts.

The lighting is more a funtion of what plants you want. Some plants will not thrive without very high light levels (3wpg or so). If you ae going for red plants like rotala macrandra and other light lovers, go with the 3.4 wpg and co2 added.

if you go with the higher light/co2 combo, make sure you have a good scissors, you will be pruning often