Max, you might want to try search SPX 860's, 900's bulbs(or something
close) or some post by Wayne. He's big on over driving and using
T-8's.
I would be as well, if I had a large number of tanks to light.
CRI= our color preception, generall the higher the number, the better
color but this does not tell you how much usable light is reaching the
plants.

K temp, plants do great at most Kelvin temps that are offered. But
most 5000K to 6500K temps are used most often. 4000K -5000K is my own
personal preference. But it is slightly yellow.
If you are planning a multible bulb set up, you can add 4000-5000K
bulb for 2-4 bulbs and 1-2 Bluer higher temp bulbs for the nicest
balance for the eye and the plants.

I've had Discus in very high light tanks(4+gal). They did not act
differently, they seemed to like it, they came out and swam all over.
They had plenty of hiding spots and did their own thing.

Either way, with 4-6 bulbs, you should do fine. You'll want to get
some nutrients for the tank besides traces only. Some K+, like K2SO4
would certainly help.
Great reflectors are worth every pound(or are you folks getting
euro'ed yet?).

If you are using soft water/RO etc, check the NO3/PO4. If low, plan on
getting some KNO3/KH2PO4 etc to raise the NO3/PO4 to good levels for
the plants(roughly 5-10ppm of NO3 and 0.4-1.0ppm of PO4). Do not over
stock/ovwer feed the tank!! 6-7 adults fully grown would be the max
number I'd keep. The waste product that causes algae is not the
PO4/NO3, it's the NH4. The filter/plants can only remove so much
before the algae sense the NH4 building up, and it does not take much
and you go an algae bloom.
Keeping the fish loads/feedings reasonable will insure a better long
term health of the fish and the tank.
Healthy plants/no algae= healthy fish.
Take care of the plants, they will give the same in return for the
fish.

Let me know if you want the links to get the KNO3/KH2PO4/K2SO4 in the
UK.
Regards,
Tom Barr