I work for Europes highest level drift series. I drive in Europes highest level drift series. I also Judge professionally accross Europe, so I think I'm reasonably well qualified to answer this.
In real life, there are several championships that allow 'Slicks' and similar and yet NO-ONE uses them. Why? Because they don't provide progressive traction and make most cars almost impossible to drive. I know a few people who have tried to use them, one had a large accident in the processs, one had a catastrophic tyre failure which caused over a £1000 worth of bodywork damage, and the other failed to qualify for the top 16 for the first time in his career. All went back to road tyres immediately.
In our main series, most drivers use normal road tyres, the equivelant of Comfort Hards, for battles and pretty much ALL of them use them for qualifying.
When things change, is in the battles. Some drivers will use super soft track tyres, Driftworks use the Federal RSR for instance, a couple of teams use the Maxxis MZ-1 Drift, I and a couple of others choose to use Kumho V70A's. This is because in the battles, any speed advantage is a good thing, but it usually has some effect on the performance in drift, reducing overall angle and smoke.
The fact is this, drifting in QUALIFYING is mostly about ANGLE and SMOKE. Therefore you use the tyre which allows the most of each (comfort tyre). Speed during solo runs is irrelevant to a point, as the two or three extra miles an hour a 'sport' tyre gives is not noticable to the judges (and in real life it IS only 2-3mph). The extra grip provided by the 'sports' means that with any given power in a drift car, you will be able to hold more angle and produce more smoke with a 'comfort' tyre because you can spin the wheels more. Using a 'Sport' tyre in qualifying will only hurt your score in 90% of championships.
So why do we use them in battles? Well, in battles, it is an accepted fact that both drivers are trying to go more quickly than in qualifying as you are trying to beat the other driver. As a result, while trying to find grip and traction, your angles and smoke levels are inevitably lower than in qualifying, this is because, as most people know, grip drops with the amount of wheelspin (more spin = less grip). Because of this, what you find is that, drifting on a 'comfort' tyre while trying to find speed produces similar results to drifting flat out on 'sports', the only difference being that you can queeze a tiny bit more speed out of the sports.
In real life, the differences in speed are in fact, quite negligable. A top level drifter on 'comforts' will be easily able to keep up with a mid level drifter on sports. The problem arises from the fact that the same IS NOT true in GT5. Sports in GT provide SIGNIFICANTLY more grip than comforts, meaning that the closing speeds on track are just massively unrealistic. In real life even slicks don't provide as much extra speed over 'comforts' as sports do in GT5. The difference is the way the tyre relinquishes it's grip and the way it recovers it. As a result, many of us who are 'pro-level' (i don't say pro as, like someone said above, none of us get paid to play GT5) will pick a tyre that MOST can drift on (comforts) and limit a room to that so that people can join in knowing that everyone is going to be the same speed and enjoy a bit of twinning.
I actually often find that most people who want to use sports are actually terrible drivers and can't link a circuit up in any way, just drifting individual corners, which frankly is pooh LOL They seem to thrive on the argument that they are 'fast' but they clearly don't get that drifting is not about outright speed. The main point being, I have NEVER found someone who can drift a course better on sports than they can on comforts.
Once you can link an ENTIRE track on comfort hard, then you progress to comfort medium, then onto comfort soft. If you can STILL link the entire track on comfort soft, THEN go and try to drift on Sport Hard's. But until you've done that, I'd be willing to bet money that a lot of the guys on comfort hard will hand you your ass in every battle you have