On that note, I remember @eggmeister posted a list of 10 routes with the shortest running time a few years ago. The (N)321 and 401 were included on that list.
Are there any other fast bus routes to do? I've kinda been put off exploring London recently simply because traffic is so bad :/
I use the night 321. It has a high average speed because it barely stops anywhere south of Lewisham and goes for fair distances without signal controls to slow it down. The B9TLs are likely to send you to sleep, many a passenger have been unceremoniously woken up at Sidcup Tesco
RandomBusesGirl has listed most of the routes I thought of, but I'll add some comments;
N8 between Bow and Hainault - a few miles of dual carriageway. You're unlikely to find LTs being pushed harder elsewhere in London.
34 - a good ride at the right times of week. I got on one at 7am on a Saturday morning once (long story). Served almost every stop on the North Circular, but that VWH was reaching 50mph between stops. Wouldn't bother when there is traffic on the road.
95 - worth a ride along the A40 if you can get an E400.
97 - all 30mph roads but there are a few drivers on there who seem to have dropped out of Formula 1 academy and ended up on the buses. Almost guaranteed a lively ride early mornings and late evenings.
107 - a fairly fast moving route with 40mph sections and decent Enviros.
113 - 40 and 50mph sections along the A41 with Enviros actually capable of reaching those speeds.
123 - 40 and 50mph sections between Gants Hill and Waterworks Corner, Woodford - better done with one of the "sporty" long VLAs, not a dead DW.
173 - never ridden this one but reckon the Topodyn equipped Enviros might provide an enjoyable ride on the A13 between Beckton and Dagenham
203 - Citaro fun on 40/50mph dual carriageways for most of its length. Most of them are limited to 40 however.
217 and 231 for a good run along the A10.
232 along the North Circular, though the drivers have always been slow when I've been on it. The bits covered by the 112 aren't as good.
258 - a few 40mph sections. Some drivers enjoy hammering the 6 speed B7TLs, perhaps enough to make you want to do a round trip.
269 - another route on 30mph roads where the buses are motored in the evenings.
290 - miles of dual carriageways in an E200 if you're into that sort of thing!
292 - runs along a 50mph stretch of the A1(M) for a mile or two.
Think the 370 is NSL single carriageway for some distance, but the road it runs along is too narrow to safely do 50mph.
422 - lively ride in the evenings.
482 - decent run west of Hounslow West with 40/50mph dual carriageways and Enviros capable of keeping pace with the traffic. You also get to ride around the perimeter of Heathrow Airport.
607 - 40mph from Hayes By-Pass to Uxbridge, but low chances of getting a fast driver - most take it easy and cruise just above 30.
A10 - the only London route to go on a motorway, it moves quickly between Stockley Park and Heathrow Airport. As mention, just try not to land a Metroline DE. The ex First DEs are a lot better.
H18 and H19 - dizzying ride around the borough of Harrow in the evenings.
U4 - you might be surprised at just how hard the buses are driven on what's been described as a "racetrack" of a route. I've rarely experienced anything else like it, the slightest dither ultimately leads to the bus getting rammed then overtaken, especially if it's an Enviro followed by a VP. The route is full of corners so you might have to hold on tight.
Night routes - iBus incentivisation killed off the ridiculously fast rides common a few years ago, though they seem to be making something of a return with high frequency monitoring. On weekends, the 35, N155 and N207 often see what feels like racing by drivers, lots of overtaking which can be entertaining so long as you're not on the bus that is getting mullered with passengers. The Uxbridge N207s are often to be found flying along.
Might want to try some rail replacements too. When on high frequency RR routes I always overtake my leaders, there are definitely a few fast drivers about.
And if you are feeling lucky, try N137 from Crystal Palace down Gypsy Hill - one driver in a blue moon will race downhill. But only one did that for me so far 
I'm amazed you've done that many night bus routes to what I guess is their full extent. You seem to be "on the buses" at all hours of the day and night. 
