Make sure the wiring and switches are all up to spec.  I can't remember if
the disco 1s use relays for the headlamps or if the current runs through the
dipswitch etc., probably depends on the age and model.  I have, however,
seen 100W headlamps cause failure of light switches and so on in the longer
term.  I think the circuitry can handle it when new, but it sure finds out
the dodgy connectors and such.  

best way is to fit heavy duty wiring and relays.

modern motors seem to have ever thinner gauge wiring, too - headlamp supply
wires used to be nice fat things, capable of taking 15A with ease; these
days, they look way too thin and are not really likely to take double the
current too well, even if they don't overheat they'll create a voltage drop
which reduces the output of the bigger bulbs.

Been somewhat impressed with Philips vision plus (or other similar from
other makers) as standard-wattage bulbs giving a good light.