Well in my opinion (sorry it's a bit late)!
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You have done the right thing.
Firstly, I booked two weeks in the Signature last September through Luxury Suites International and I saved $1000 on what MGM were asking directly. I can't remember the exact figure but I'm sure it cost me something like £1100 for the two weeks in the Sig. I stayed in a Junior suite, which, by the way is lovely and certainly more than adequate for a couple. I've put a review on it in the Hotel section on here.
To get that price, I booked initially through MGM themselves.......then cancelled when I found a good deal with LSI. Then, 2 weeks before we were due to go, LSI dropped their prices again by another $500 or so. I called them directly and asked if they could adjust my price and they said to cancel my first booking and rebook the second deal. I found them to be excellent, with comms being very very fast between us and I had a few questions that were answered very quickly too.
One thing to bear in mind is that the Signature use a completely different computer system to MGM Grand, so don't contact MGM for info on the Sig. If you get a players card for MGM, you won't be able to use it against your Signature room, I could only get free buffet deals from them.
For your dates, LSI will currently do a Junior suite for the 20-28th Sep for $1462 (£1082)
When I first spoke to LSI, I asked them if they would do me an extra discount as I was staying for two weeks - they did this. They will also offer a discount for a 7 or 8 days stay so be prepared to call them directly to haggle with them. Remember, 75% of the Signature is privately owned and everyone wants to make money so they will be willing to drop a bit.
If you were to eventually book through LSI, don't worry if you don't get confirmation immediately......I panicked last year as I didn't get conf. so I called LSI and they said that the room is confirmed 10 days before you arrive. I had no issues whatsoever.
Congratulations and have a great trip.........and remember to come back and write a trip report!!
Cheers,
TC