Ive got a fair engine and various complimentary mods - and I can tell you for sure that the top speed on my 16v doesnt sound any faster than you guys.
Like the Captn says, its down to drag - and Im sure I need not remind you that the 16v has a huge scoop to gulp in air and lots of various other air holes (like next to the fog lights, if you take the blanking plates out).
In my experience, my car will belt it to 120, but takes ages to get to top speed - 130 in my case. If theres a hill involoved, then my car sticks at 125 tops. Mat Brown also has a 170bhp conversion, but he has the 2.0 bottom end. That definitely had more speed than I did as we found out a few months ago - but thats probably due to torque, as mine slowed to 125 on an incline, while he ploughed past to make 130+. Still more than enough for us both to waste an MG ZR 160bhp!
In any case, top speed is something I very, very rarely use. With the 16v its really the blinding acceleration from rest and in gears (like 30-70) that you use - to devastating effect. Something like a 5 Turbo is still slightly better than a 16v for 0-60 (but only by a cars length), but once youre over the initial inertia, the 16v is a quicker car in standard form.
Its an expensive business moding a 16vs engine, so Id have a hard think before you splash out - unless your engine blows! Coming from an RT anyway youll probably be more than happy with a standard 0-60 of 7.7 and top speed of 130mph!!
Autocars test in 1992 got 0-60 in 7.4, 30-70 in 7.1 () and top speed of 127mph. The only area that the Williams test was better at was top speed in 1993 (130mph).