If the turbo is sized properly you can build plenty of low speed torque. Think diesel. Also there are some pretty neat modern tubocharged gas engines out there. The manufacturer I work for has a 2.3 liter 4cyl that is at peak boost of 13.5lbs by about 1900 rpm. Peak torque is 260 lb ft at 4500 rpm, but the engine is at 90% or more of peak torque from 2000-7000 rpm. Pretty impressive for an engine of only about 140 cubic inches. Of course this is achieved with a variable inlet turbo and modern electronic engine controls. I would think if you sized the turbo to build boost and torque at your desired RPM you could make some impressive gains. The main tradeoff would be the lack of that big turbo push at higher RPM,s because of the restrictive inlet needed to produce the low speed boost would create a drop in boost as RPM increased. The main concern would be the detonation that comes with high boost levels and engine loads at a low engine speed. This would be very difficult to control in a relatively ancient engine design even with modern (computer) controlled boost, fuel, and spark.
