Hello mate!
Unfortunatly from an insurers point of view unless you have an agreed value on your vehicle then the insurers will only pay you what they see as the "book value" regardless of what you tell them the vehicle is worth (check your documents, where you have the value it will normally say market value or
maximum value £1500 not minimum), and on an 87 GTI then unfortunatly i would be surprised if it was much more than £500.
On any GTI i would recommend you get an agreed value (i know adrian flux offer these as well) and that way at least you will be definatly pait out what you or i or another enthusiast would pay for the car but without seeing it myself its a bit hard to give an exact opinion on the value.
With regard to TPFT v Comp its really up too you, the way i look at it is to add up the cost of the insurance and the excess and then compare that to the value of the car eg if your insurance for comp was £1000 + an excess of £470 then you would be paying out £1470 to get back £1500 in the event of a claim.....for the sake of £30 prob not worth so may as well just save the money and go TPFT, it but depends how much they have quoted you. However you do get added benefits such as windscreen cover etc on comp.

Obviously these are just my
personal opinions so its up to you! (so if you go TPFT then stick your car in a ditch dont come to me

)
Hope this helps (and makes a bit of sense) a bit, let me know if you need any further info!
Martin