I remember reading somewhere about putting foil tape on the top half of tubes like this to act as a reflector. Think this would be worth the cost of foil tape on NEW bulbs? If anything would benefit from it I would think the big T12s would, but, the light fixture and ceiling is already painted white...
I thought two T8 4' bulbs would make as many lumens as one T12 8', but that's about 5700 vs 8800 lumens. These old buzzers are becoming more attractive...
Two F32T8s
would be as bright as a F96T12, except for the fact that yours are high output. Normal F96T12s are 75 watts (60 if a Watt-Miser version), and the High Output ones are 110 watts (95 if Watt-Miser).
How well is the space currently lit? If it is currently well-lit enough for you, I would recommend one of two solutions (I am assuming the current fixtures are strip lights). The first is, if you do not mind staying with 8' tubes, would be to go to F96T8/HO. They put out about the same amount of light as a F96T12/HO, but use 86W instead of 110. If you want to go to 4' tubes, either put about 4 F32T8s for each F96T12/HO, or, if your ceilings are high enough where this will not cause excessive glare (F54T5 lamps have a very high surface brightness), put about 2 F54T5/HO for each F96T12/HO. A F54T5/HO puts out about 4600 mean lumens, and consumes 54 watts. Downside of this approach is you will have to go to a supply house to get fixtures and lamps.
In case you are worried about the difficulty of changing 8' lamps, don't be. Yes, you do have to be more careful with them because the are 8' long glass tubes, but you don't have to mess with turning them when you install them like you do with a 4' lamp. On one end of the fixture, the lampholder is a spring loaded plunger. The other one is fixed. You first insert one end of the lamp into the spring loaded lampholder, press it in (compressing the plunger), and then stick the other end in the other lampholder. All types of eight foot lamp are like this.