i can understand why they want that much but no, i wouldn't pay that.
i used to keep credit card and passport in the till until they returned the bike after confirming their ID. if i was paranoid because they weren't regular i'd ask for £500 (for a 2 grand bike). this is assuming that when you say testride you mean taking the bike from the shop to ride your own trails. regular customers i knew well i'd let them have the bike no deposit.
i found that asking for £500 up front for the demo bike put off the time wasters but the serious guys had no issue with it. most people who want a testride tend to have decided which shop they are buying from so i used to say put down the deposit take the bike for the weekend, ride it, if you like it and want to order i'll use the £500 as deposit, if you don't like it then it's on to the next testbike.
at the end of the day they are giving you a very expensive bit of kit and it can't be traced if they are nicked like cars etc, meaning the police don't give a crap so asking for £1500 would cover the cost of the stolen bike at trade price.
i can understand you not wanting to put down £1500 but they may be thinking that if you can't put down £1500 then why is he looking at £1500+ bikes, he must not have the money, therefore they may think should they bother with you.
i am not saying that attitude is right but it may be a possibility.
it might be worth saying to them you are not comfortable putting down £1500 because you'd feel tied to them if you don't like the bike, maybe offer £500 to show you are serious about buying.
