the wheel and tyre package is only as good as the suspension package. the better you can set up the rear end the better everything will work together. slicks tend to be a more forgiving than a radial.
was talking to Khain with the purple Holden HR recently, he changed from the 3754R ET Street 275 to the 3754x x275 Radial Pro he 60'd a 1.25 on the weekend, best before with the ET Street tyres was a 1.30, though he did also change converters recently so that is another factor, car weighs 3600lbs and is on a 15x10 as well if i remember correctly. he loves them better than the ET Street.
me personally, can't compare between the 2 radial tyres, but, can compare between the radial and a slick. we run the radials on a 15x10 from what i have seen with other people, at high speeds in the deep end they tend to get a little squirrely on a 15x8 or 15x9, that would be even exaggerated worse on a 4000lbs car.
the Radial Pro is a little taller has a 89'' circ vs 87'' of the ET Street and has 9.8'' vs 9.5'' tread, plus with the more slick like grooving, the Yanks are 60'ing a 2-3/100ths quicker with the Pro. the Pro is also 1-2lbs lighter.
we run the ET Street 3754R on a 15x10, as it's more like a street tyre and because we drive on it to the track when racing etc. absolutely love the tyre, out of 50 passes we have only spun the tyres once out of the hole, the rest have been all clean passes, 60' a 1.300 on the radial, a 1.285 with a slick, however, when we run a slick the car is usually no exhaust/100lbs lighter, with the exhaust the best the car has ever 60'd with a slick is 1.32, so really the radial is showing a better result. the good thing is all this testing is done with the same fuel/tune. we have also got more runs out of the radial then a slick, around 40 passes the slick starts to go a little how you going, whereas the radial we have done 50 passes, 95% have been within the 1.30-1.34 60'. in the same trim with slicks but 50lbs lighter the car has run a best of 8.30@166mph, has run a 8.31@165 on a radial. though that slick time time is also 18 months before and the car was 50lbs lighter with slick, because of the lighter tyre/wheel and had a fibreglass bonnet.
we haven't even begun pushing the radial, i believe there is a 1.27 in it for sure, we leave the line on 5500rpm/12-13psi with a 10.5'' slick we leave at the same rpm but at 15-16psi, we could definitely leave the line alot harder with the radial, plus, the car being 100lbs heavier isn't helping
if you are driving the car i wouldn't hesitate saying go the ET Street 275, but, if you don't mind taking the wheels off at the track the 275 Radial Pro on a 15x10 it comes down to how badly you want to achieve the best time you can.