I just checked my documentation. The limit was the same as the base policy; $10,000,000. Here is the actual policy provision (sorry for the messy layout - I didn't want to play with it):
Unstable Pre-existing Medical Condition Coverage
This Optional Coverage can be purchased as an endorsement to an Emergency
Medical Insurance Worldwide or Worldwide Excluding USA plan.
Benefits
Maximum limit − Up to the Policy limit
This coverage does not increase the maximum limits of the Emergency Medical Insurance
plan stated in the Policy.
When this Optional Coverage is purchased, we will pay for the reasonable and customary
charges for medical and related expenses up to the coverage limit for an acute, sudden
and unexpected emergency medical condition incurred as a result of your pre-existing
medical conditions that were not stable and existed on or before the date of departure.
The charges must result from an emergency that first occurs after coverage commences
(including after any applicable waiting period) and while you are travelling outside your
province/territory of residence.
Coverage is subject to the terms and conditions specified in the section entitled
Emergency Medical Insurance, except for the wording under the sub-heading Pre-existing
Medical Condition Stability Exclusion and is also subject to the exclusion described below.
Exclusion
In addition to the Emergency Medical Insurance exclusions and to the General Exclusions
shown on page 47, we will not be liable to provide coverage or services, or to pay claims
for expenses incurred directly or indirectly as a result of:
Any medical condition which is not stable on or within the 7 days before the date
of departure.