Have speedo... will geek about numbers.

OK, first ride to work with the new speedo fitted and it looks as though my cruising speed is about 40kph. Up from a little under 30kph on my upright.
I also tried a different route, a somewhat shorter one that's mostly on better roads but has a nasty bit of traffic at the end. But if makes a real difference being able to sit at 40-45kph in traffic, since that is roughly the speed all the motorists were doing in the city. Which means I can get to work in about 25 minutes if I want to.
The ride through Royal Park/ along Royal Parade was nice, good seal and slightly downhill with a decent bike lane. Which does bring up an important question - what should I do about cyclists who doze along on the right hand edge of the bike lane and don't respond to being pinged at? The cyclist in question seemed quite upset that I whipped past on his left (at least a metre to his left!). Do I need a big loud piezo horn like the race teams use or just get used to slowing to 15kph and waiting while the cyclists wakes up, looks around and (if I'm lucky) moves left?
Currently I have a wee pingping bell (visible here) attached to the hatch so it's outside but I can reach it while riding. It's also in fairly dirty air, so it's not costing too much. But it's close to my ears so I'm reluctant to mount a proper bell there, and the only alternative I can think of is an electric horn (I already have 12V for my lights). But that's not a noise people associate with bikes, so it'd be as bad as an air zound. The "honk honk" ... obstacle looks around dazedly going "I wonder what made that noise", rather than leaping sideways (and often looking aggreived).
That last point is one that also concerns me: some pedestrians are offended if cyclists don't ring just to say they're coming through, but others are offended at being pinged at (I assume they interpret it as the imperitive "get out of my way"). On thje Upfield Path this is compounded by a few spots where there are pedestrian entry points at building corners, so it's wise to ping madly as you approach from the blind side, as there's literally no time to do anything if a pedestrian steps out at the wrong moment. So it's a bit of a no-win situation. Always ping, and offend quite a few people; never ping and shock others while offending a fair few more; or ping only when you want someone to move aside (offending some and perpetuating the "if I ping I want you to move" meme).