The G-Force's can be driven fast, but only if you are skilled enough to keep the car within a very small range of slip angle.
I have a friend who tests tires for Michelin/BFG, and he admits that Hoosiers and Goodyears are easier tires to drive faster, but the BFG's can be driven as fast with the right know how.
Since they were so hard to drive though, BFG has stopped production. Except for remaining inventories, they are NLA. There is a rumor that Michelin has a street tire in Europe with as much grip as the G-Force R Compound...according to my source, those tires could see our shore in the next few years.
Even if the BFG's were still available, they are not a tire for rookies. I used them on my car when I drove in BMW CCA Club Racing, but I won't be using them when I start out in ITS (even though I can get some of the remaining tires for really cheap), as I'll be getting accustomed to having no ABS. The BFG's will flatspot just looking at them.
For a rookie tire, its hard to beat the Toyo's and Yokohamas. The Kumho's are a good next step up and they are what I'll start my ITS season on. Once you get proficient in the car and are looking for the last couple tenths, its time to invest in Hoosiers of Goodyears in my humble opinion.