The distance and multiple stops mean it's an hour each way to Liverpool Street, compared to 30 mins. LGW to Victoria or London Bridge. That makes the journey comparatively long and slow in my book.
Any time saved clearing the airport is likely to be lost waiting for the next train as they materially less frequent than at LGW. I fly IOM - LGW 30+ times a year and, without bags, it takes 7 minutes from gate to station platform.
Open up IOM-SEN 3 or 4 times a weeks as another London option by all means, but that number of seats is greater than the entire IOM - London market and could f*ck everything up.
If this is for real, it will only end in tears at Reynoldsway........