From what I have seen, GT5 uses the same non-tyre-pyshics tyres, at least Forza uses Pacejka (sp?) tyre tables (even though they admit some if it had to be fudged because the cars didn't "feel" right).
Some interesting info on the Forza physics >
http://download.microsoft.com/ ... 4f54c/Forza2MNL_LE_EN.pdf
to para-phrase
"We tried different tire models after our “green light” demo. We kept tuning
the physics model, finding bugs, and working on it. New cars exposed new
issues. We had two basic ways of expressing the tire physics. The traditional
way that simulation racing games do this is Pacejka’s Magic Formula—no joke,
that’s what it’s called. It’s got a whole slew of variables you tune and input,
and it spits out friction values. But the cars didn’t control the way we wanted
and the results didn’t mesh with a lot of our real-world data. I’ve had enough
experience tuning with that formula that I can feel it at work when I play some
racing simulations. It’s not quite right, but it’s close. Toyo put me in contact
with their tire engineers. It was hard to understand the data they were giving
me, and I had to ask a lot of questions, but they slowly brought me up to
speed to where I could understand what their graphs and data were saying.
Pacejka’s formula is close, and really good in most situations, but not all, and
not for transitions between states. It just doesn’t feel right.
So we went to what could best be described as a table system. We have a table
that linearly interpolates between the curves we have for different weights, tire
pressures, and other variables. It’s a very big, computationally difficult system.
That’s what was nice about Pacejka’s formula when we were using it—it’s
very lightweight, where our approach is extremely heavy. That’s why in Forza
Motorsport, drifting, for example, is very real and very responsive. Drifting is
all about weight transition, and you can control it with your throttle in Forza
Motorsport because in our model the movement between those curves is very
smooth and precise. We spent a lot of time getting it right. The tire model
is amazing, but there are always things to improve. There are things in our
physics model I’ll want to improve forever.
Forza Motorsport 2 is a simulation, not a complete emulation—no one has
ever done that, no matter what they claim. We can’t completely emulate tire
technology until the scientists learn it, and they haven’t learned it all yet. Tires
do some funky things. They’ve got load sensitivity, which involves non-
Newtonian physics. A tire with a coefficient of friction of 1.0 at 500 kg actually
develops a smaller coefficient of friction at 1,000 kg. With 500 kg on the tire,
it might require a value of 500 kg force to push it. But with a 1,000 kg load, it
might require a value of 800 kg force to push it. Understanding this is huge in
understanding how tires function. It’s a big deal to simulate this, though few
people know about it. When I told the tire engineers, they were amazed we
were doing it."
maybe it is how you had it setup? If you have Overall Effects Strength set to your preference and all the other settings to zero or off it should be ok on the momo. The G25 does feel more responsive though. And the G25 actually feels like a wheel instead of a toy 
