Have you had a recent tune up? That seems too slow to me, even with that 2300cc four cylinder. Have you actually taken it down the freeway and timed your travel between milemarkers to check speedometer accuracy?
I have owned W114/115 sedans with that engine and carb. I like the idea of changing to a Weber Carb, but make certain you are fully tuned and adjusted before concluding you need to change carbs.
Tires make a huge difference. The smaller, the more torgue, just as Dai noted. My first 300GD had stock tires (29 inch), and I thought it would be cool to put on 32 inch tires. That lasted one day, and I exchanged them for new 29.5 inch tires. I was happy again, even though I never drove faster than 100 kph. I was a happy owner. It is all about the mental state that we bring to these vehicles. Tires and cars are a lot like dogs and purpose, and a happy little miniture Schnauzer is going to be a major league hunting bird dog. But, that is just fine with me.