The old thermal flasher units would flash quicker with less load (a bulb out) not slower, this "warning feature" was more of an inherent design limitation of the technology than an intentional design feature, a lucky accidental bonus with limitations, hook up a trailer and the flash rate would often be too slow.
Obviously fitting bulbs of different wattage to standard would effect the flash rate.
So imagine this scenario, you're on a trip some where and you notice that a bulb has blown, you go to the only garage for miles, and they don't have any bulbs of the wattage required for you application, you use what you can get as it's better than nothing. The result could be that you now have a flash rate that doesn't meet the legislative requirements.
Normally this wouldn't matter much, but if you to encounter a copper having a bad hair day and a particularly large amount of shit on the liver, he might notice this out of spec flash rate and give you a defect notice (called a canary in these parts because they stick a bloody great yellow defect notice sticker on your windscreen) Most coppers wouldn't notice or even bother hassling you if they did, but there are some real pricks out there.
Bin the shitty old type flasher unit and go for one that will work with your LEDs if you must have them, I see no point in fitting LED bulbs in the indicator circuit, if you have to go to the trouble and expense of fitting ballast resistors into the circuit.
It defeats the purpose of lower wattage LED bulbs if you have to fit ballast resistors to up the wattage just to get the damned things to flash!
I can however see one possible advantage of using LEDs (without ballast resistors) their lower wattage requirements would put less load on the shitty indicator switches used in our cars, as would the binning the thermal flasher can (damned things use a fair bit of current themselves)
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"
'80 Triumph TR7, '73 Land Rover (Ford 351. V8),
'97 Ford Falcon Longreach 'S' ute,
'98 MG-F, '69 Ford F250.